These are a soft peanut butter cookie base baked in a 9×13 pan, covered with marshmallow while still hot so it melts into a smooth white layer, then finished with a drizzle of warm peanut butter. They cut into twelve generous squares.
The timing on the marshmallow layer is the whole recipe. Added to a hot base it spreads and settles into an even sheet. Added a few minutes late it tears and clumps.
Total hands-on time is about fifteen minutes, though the bars need a proper cooling period before they will cut cleanly.
Why You Will Love These Bars
Three distinct layers, one pan. Cookie, marshmallow, and peanut butter drizzle, with no separate baking steps.
The oven does the marshmallow work. Residual heat melts it. There is nothing to boil or temper.
They travel and stack well. Once fully set, they hold their shape in a container.
Twelve big bars from one pan. Cheap per serving and enough for a crowd.
No mixer required. A wooden spoon and a bowl are enough.
Recipe Overview
Prep time: 15 minutes
Bake time: 22 to 25 minutes
Cool time: 2 hours
Total time: About 2 hours 40 minutes
Yield: 12 bars
Difficulty: Easy
Best for: Potlucks, bake sales, lunchboxes, parties, after-school treats
Ingredients
For the Peanut Butter Base
• 1 cup (225 g) unsalted butter, softened
• 1 cup (200 g) packed light brown sugar
• 1/2 cup (100 g) granulated sugar
• 1 cup (250 g) creamy peanut butter
• 2 large eggs, room temperature
• 2 teaspoons vanilla extract
• 2 1/2 cups (310 g) all-purpose flour, spooned and leveled
• 1 teaspoon baking soda
• 3/4 teaspoon kosher salt
For the Marshmallow Layer
• 1 jar (7 oz) marshmallow creme, or 4 cups mini marshmallows
For the Drizzle
• 1/2 cup (125 g) creamy peanut butter
• 2 tablespoons unsalted butter
Choosing the Right Peanut Butter
Commercial creamy peanut butter is the correct choice here. Brands like Jif and Skippy contain emulsifiers that keep the fat suspended, which gives a stable base and a drizzle that sets rather than separates.
Natural peanut butter is the one substitution that causes problems. The oil separates, and even well-stirred it makes a greasier base and a drizzle that pools rather than sets. If it is all you have, stir it thoroughly and chill the bars before cutting.
Crunchy peanut butter works in the base and adds texture. Use creamy for the drizzle so it flows evenly.
Do not use reduced-fat peanut butter. The fat is replaced with starch and sugar, which makes the base dry and crumbly.
Almond or sunflower seed butter can be substituted for allergies, with the same commercial-versus-natural caution.
Why the Marshmallow Goes On Hot
The residual heat of the base is what turns a stiff jar of marshmallow creme into a smooth spreadable layer.
Work within about two minutes of the pan leaving the oven. Marshmallow creme softens on contact with a hot surface and can be spread with an offset spatula in a few strokes.
Wait five minutes and it tears. The base cools quickly at the surface, and cold marshmallow drags rather than spreads, pulling up crumbs of cookie with it.
Mini marshmallows are the easier alternative. Scatter them over the hot base and return the pan to the oven for 2 to 3 minutes until puffed, then spread with a buttered spatula.
Butter the spatula either way. Marshmallow sticks to everything, and a lightly buttered blade slides across the surface.
How to Make Peanut Butter Marshmallow Bars
Step 1: Prepare the pan
Preheat the oven to 350°F (175°C). Grease a 9×13 inch pan and line it with a parchment sling, leaving the ends hanging over the long sides.
The sling is what lets you lift the whole slab out and cut it on a board, which gives far cleaner edges than cutting in the pan.
Step 2: Cream the butter and sugars
Beat the softened butter, brown sugar, and granulated sugar together for 2 minutes until light and fluffy.
Step 3: Add the peanut butter and eggs
Beat in the peanut butter until smooth, then the eggs one at a time, then the vanilla.
Room temperature eggs matter. Cold eggs cause the mixture to curdle and the finished bars to bake unevenly.
Step 4: Add the dry ingredients
Whisk the flour, baking soda, and salt together in a separate bowl, then add to the wet mixture and stir just until no dry flour remains.
Stop as soon as it comes together. Overmixing produces tough, dense bars.
Step 5: Press into the pan
Spread the dough evenly in the prepared pan. It will be thick and sticky.
Wet your hands or use a piece of parchment to press it flat. It will not spread on its own.
Step 6: Bake
Bake for 22 to 25 minutes, until the edges are golden and the centre looks set but still slightly soft.
Underbake slightly rather than overbake. The bars firm up considerably as they cool, and a fully firm centre in the oven means dry bars on the plate.
Step 7: Add the marshmallow immediately
Remove the pan from the oven and spoon the marshmallow creme across the hot surface in dollops.
Spread gently with a buttered offset spatula, working from the centre outward, until you have an even white layer.
Step 8: Make the drizzle
Melt the 1/2 cup of peanut butter with the 2 tablespoons of butter in a small saucepan over low heat, or in the microwave in 20-second bursts, stirring until smooth and pourable.
Step 9: Drizzle and cool
Drizzle the peanut butter mixture over the marshmallow layer in lines using a spoon or a fork.
Cool completely at room temperature for at least 2 hours before cutting. This is not optional if you want clean squares.
Tips for the Best Bars
Use the parchment sling. Lifting the slab out and cutting on a board produces sharp edges that cutting in the pan cannot match.
Butter your spatula for the marshmallow. It is the difference between spreading and dragging.
Underbake by a minute or two. These bars continue cooking in the hot pan.
Cool fully before cutting. Warm marshmallow smears down the sides of every cut and ruins the layers.
Wipe the knife between cuts. A hot damp cloth on the blade between each cut keeps the marshmallow from building up.
Common Mistakes to Avoid
Using natural peanut butter. Oil separation makes a greasy base and a drizzle that never sets.
Waiting to add the marshmallow. Two minutes is the window. After that, use the mini marshmallow method with a short return to the oven.
Overbaking the base. Peanut butter bars go from soft to dry very quickly at the end.
Cutting while warm. The marshmallow layer stretches and tears rather than cutting.
Cooling in the refrigerator. It seems faster but makes the base hard and the marshmallow rubbery. Room temperature is correct.
Easy Variations
• Chocolate drizzle. Melt 1/2 cup of chocolate chips with a teaspoon of oil and drizzle alongside the peanut butter.
• Fluffernutter with chocolate chips. Fold 1 cup of chocolate chips into the base dough.
• Salted. Sprinkle flaky sea salt over the drizzle before it sets.
• Rice crispy topping. Stir 2 cups of crisp rice cereal into the melted peanut butter drizzle and spread it over the marshmallow as a full top layer.
• Nutty. Press 1/2 cup of chopped roasted peanuts into the drizzle while it is still soft.
• Smaller batch. Halve everything and bake in an 8×8 pan for 20 to 22 minutes.
What to Serve With Them
A glass of cold milk is the traditional and honestly the correct pairing.
Coffee balances the sweetness better than anything else for adults.
For a dessert plate, serve a bar slightly warmed with vanilla ice cream.
At a bake sale or potluck, cut them smaller than twelve. These are rich, and a half bar is often enough.
Make-Ahead and Storage
Make-ahead: Bake and assemble a day ahead. The bars cut most cleanly after a full night at room temperature.
Room temperature: Store in an airtight container with parchment between layers for up to 4 days.
Refrigerator: Not ideal, since it hardens the base, but it does make cutting easier. Bring back to room temperature before serving.
Freezer: Freeze cut bars wrapped individually for up to 3 months. Thaw at room temperature for an hour.
To reheat: These are not really a reheating dessert, but 10 seconds in the microwave softens a bar nicely.
Frequently Asked Questions
Can I use mini marshmallows instead of creme?
Yes. Scatter 4 cups over the hot base, return to the oven for 2 to 3 minutes until puffed, then spread with a buttered spatula.
Why are my bars dry?
Overbaking, or too much flour from scooping the measuring cup. Spoon and level the flour and pull the pan while the centre still looks slightly soft.
Can I use natural peanut butter?
It works but produces a greasier, more crumbly result. Stir it very thoroughly and expect a softer set.
How do I get clean cuts?
Cool completely, lift the slab out using the parchment, and wipe the knife with a hot damp cloth between every cut.
Can I make these gluten-free?
Yes, with a cup-for-cup gluten-free flour blend containing xanthan gum. The texture is slightly more crumbly.
Can I halve the recipe?
Yes. Use an 8×8 pan and reduce the bake time to 20 to 22 minutes.
Nutrition Information
Approximate values per bar, based on 12 bars:
• Calories: 545
• Total fat: 31 g
• Saturated fat: 13 g
• Carbohydrates: 58 g
• Sugar: 36 g
• Protein: 11 g
• Sodium: 320 mg
Nutrition is estimated and will vary with brands and portion sizes.