These Marshmallow Peanut Butter Brownies are a decadent treat that combine rich, chewy chocolate brownies with swirls of marshmallow fluff and a creamy peanut butter filling. Adapted from Sarah Kieffer’s cookbook 100 Cookies, this version makes a crowd-pleasing tray of bars perfect for baking for friends, parties, or an indulgent weeknight dessert.
Table of Contents
- 6 Reasons Why You Should Make This Recipe
- About the Author: Sarah Kieffer
- Baking Tools Needed
- How to Make This Recipe
- Recipe FAQs
- My Top Brownie-Making Tips
- Need More Brownies in Your Life?
- Marshmallow Peanut Butter Brownies Recipe
6 Reasons Why You Should Make This Recipe
- Rich and chocolatey brownies with a texture that balances fudgy and slightly cakey.
- No stand mixer or hand mixer required—simple tools get the job done.
- Swirls of peanut butter filling and marshmallow fluff add texture and flavor contrast.
- Easy to prep and bakes in about 26–32 minutes.
- Chocolate, peanut butter, and marshmallow create a classic, irresistible flavor trio.
- A trusted recipe adapted from Sarah Kieffer’s cookbook 100 Cookies.
About the Author: Sarah Kieffer
Sarah Kieffer is well-known for her cookies and brownies, including the viral pan-banging cookie technique. In 100 Cookies she presents a wide variety of cookie recipes, and she also revisits her favorite brownie formula from The Vanilla Bean Baking Book. The brownie base for these marshmallow peanut butter bars comes from that tried-and-true recipe.
Her work and recipes are popular among home bakers for being reliable, flavorful, and approachable. If you enjoy brownies and cookies, Sarah’s books and blog are great sources of inspiration.
Baking Tools Needed
These are the recommended tools for making marshmallow peanut butter brownies:
- 9 x 13-inch baking pan. A metal pan works well for even baking.
- Baking spray, butter and flour, or shortening and flour. For greasing the pan.
- Parchment paper. Lining the pan with a parchment sling makes removing the brownies easy.
- Mixing bowls. At least three bowls: small for dry ingredients, medium for wet, and another for the peanut butter filling.
- Small heavy-bottomed saucepan or microwave-safe bowl. To melt chocolate and butter together.
- Whisks and spatulas. For combining ingredients and scraping the batter into the pan.
- Wire cooling rack and a large knife. For cooling and cutting the brownies.
How to Make This Recipe
The steps below outline the process; the full ingredient list and detailed instructions are in the recipe card further down.
Peanut butter filling: Combine creamy peanut butter, powdered sugar, room-temperature unsalted butter, vanilla, and a pinch of salt in a small bowl until smooth. Set aside.
Dry ingredients: Whisk the all-purpose flour, salt, and baking powder together in a small bowl.
Wet ingredients: Whisk eggs with granulated sugar, brown sugar, canola oil, and vanilla in a separate bowl.
Melt chocolate and butter: In a small heavy-bottomed saucepan over low heat (or carefully in the microwave), melt the butter and chopped bittersweet chocolate, stirring frequently until smooth. Off the heat, whisk in the cocoa powder.
Combine: Add the chocolate mixture to the egg-sugar mixture and whisk until smooth, then fold in the flour mixture until just combined.
Assemble: Pour the batter into the prepared pan. Dollop the peanut butter filling and marshmallow fluff across the top, alternating the two. Drag a butter knife through the surface to create swirls.
Bake and cool: Bake at 350ºF (180ºC) for 26–32 minutes, until the edges are set, the top looks glossy and slightly crackled, and a toothpick inserted into the center comes out with moist crumbs. Cool completely on a wire rack, then lift out with the parchment sling and cut into bars.
Recipe FAQs
Yes. Heat in the microwave for 45–60 seconds at medium power, stir, and repeat in short bursts until fully melted and smooth. Then whisk in the cocoa powder.
Yes. Store in an airtight container at room temperature for up to 2 days. Refrigerating will extend their life by a day or two, though the texture will be firmer.
My Top Brownie-Making Tips
Use a parchment sling. Lining the pan with parchment makes it easy to lift the cooled brownies out for cutting.
Make the peanut butter filling first. Having it ready lets you dollop it immediately after the batter is in the pan.
Warm the marshmallow fluff briefly. Fifteen seconds in the microwave softens the fluff and makes it easier to dollop and swirl.
Swirling technique. Gently skim the surface when creating swirls to keep the marshmallow visible on top; over-swizzling can push it inward, which still tastes great but hides the decorative peaks.
Need More Brownies in Your Life?
The Best Salted Pecan Pie Brownies
Pumpkin Frosted Fudgy Brownies
Better Than Box Mix Fudgy Brownies
Double Chocolate Mint Brownies
Marshmallow Peanut Butter Brownies
Ingredients
peanut butter filling
- 1/2 cup Creamy Peanut Butter
- 1/4 cup Powdered Sugar
- 2 tbsp Unsalted Butter, room temperature
- Pinch of Salt
brownies
- 1 cup + 2 tbsp All-Purpose Flour
- 3/4 tsp Salt
- 1/2 tsp Baking Powder
- 4 Large Eggs, room temperature
- 1 1/2 cups Granulated Sugar
- 1/2 cup Brown Sugar, packed
- 1/2 cup Canola Oil
- 2 tsp Pure Vanilla Extract
- 8 tbsp Unsalted Butter
- 8 oz Bittersweet Chocolate, chopped
- 1/4 cup Dutch Process Cocoa Powder
- 1/2 cup Store-Bought Marshmallow Fluff
Instructions
peanut butter filling
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In a medium bowl, mix together the peanut butter, powdered sugar, unsalted butter, vanilla, and salt until combined and completely smooth. Set to the side.
brownies
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Adjust an oven rack to the middle position and preheat the oven to 350ºF (180ºC). Grease a 9 by 13-inch baking pan and line it with a parchment sling.
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Whisk the flour, salt, and baking powder together in a small bowl.
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In a separate bowl, whisk together the eggs, granulated sugar, brown sugar, canola oil, and vanilla until smooth.
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Place the butter and chopped bittersweet chocolate in a small heavy-bottomed saucepan over low heat and melt, stirring frequently until smooth. Remove from heat and whisk in the cocoa powder until combined.
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Add the chocolate mixture to the egg-sugar mixture and whisk until smooth. Fold in the flour mixture until just combined.
assembling the marshmallow peanut butter brownies
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Pour the brownie batter into the prepared pan. Dollop the peanut butter filling and the marshmallow creme over the top, alternating between the two. Drag the tip of a butter knife through the batter to create swirls.
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Bake until the edges are set, the top is glossy and beginning to crackle, and a toothpick inserted into the center comes out with moist crumbs, 26 to 32 minutes.
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Transfer the pan to a wire rack and let cool completely. Use the parchment sling to lift the brownies from the pan, cut into bars, and serve. Store in an airtight container at room temperature for up to 2 days.
Notes
- Adapted with permission from Sarah Kieffer’s 100 Cookies cookbook.
- Use a parchment sling. It makes removing and cutting the brownies much easier.
- Warm the marshmallow fluff briefly. Fifteen seconds in the microwave softens it for easier dolloping.
- Microwave alternative for chocolate and butter: Heat in 45–60 second bursts at medium power, stirring between blasts until melted and smooth, then whisk in the cocoa powder.
- Prepare the peanut butter filling ahead of time. That way you can dollop it immediately after pouring the batter into the pan.
Nutrition
Nutritional information provided above is an estimate only and will vary based on specific ingredients used.
