Decadent marshmallow brownies have a fudgy brownie base topped with gooey marshmallows and chocolate frosting. A rich and easy dessert made with box mix!

Marshmallow brownies stacked on top of each other.

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Marshmallow Brownies with Box Mix

If you are a fan of my s’mores bars or Mississippi mud pie cake, then this dessert is for you! It’s the ultimate indulgence and is made with double chocolate and fluffy marshmallows.

I remember having these marshmallow brownies (also known as Mississippi mud brownies) growing up at a friend’s house. It was love at first bite from the rich fudge brownie base to the gooey marshmallow topping and chocolate buttercream frosting!

Marshmallow brownies stacked three tall.

Ingredient Notes

  • Boxed brownie mix: For the base, I prefer to save time and make marshmallow brownies using box mix. You can make my homemade brownie recipe if you prefer.
  • Marshmallow layer: Mini marshmallows make up the 2nd layers and are puffy and gooey. Optionally, top with homemade marshmallow fluff and skip cooking.
  • Chocolate frosting layer: Last, these are topped with a chocolate buttercream made with butter, powdered sugar, cocoa powder, milk and vanilla.

How to Make Marshmallow Brownies

How to make marshmallow brownies in six steps.

Find the full printable recipe with specific measurements below.

  1. Brownies. Prepare the brownie mix according to the package directions. These are going to be fudgy chocolate brownies that are soft and moist. Mix the brownies with water, oil, and large eggs according to the box package.
  2. Bake. Pour the brownie batter mixture into a greased 9×9 inch pan and bake for 30 minutes, or until almost done.
  3. Mix. Beat the frosting ingredients in a large bowl with an electric mixer until smooth and creamy. You can use a whisk too.
  4. Marshmallows. Remove the brownies from the stove, then evenly sprinkle the mini marshmallows on top. Bake for about 3 minutes, or until the marshmallows have puffed up. Remove from oven and let cool.
  5. Spread. Place a few spoonfuls of the frosting on top of the marshmallow layer and spread with a spatula.
  6. Slice and serve. I like to cut the brownies in 5×4 rows with a plastic knife. Make them as big or as little as you need.

Baking Tips

Follow these helpful baking tips to make the best marshmallow brownies!

  • Line pan. Just like my chocolate hazelnut Nutella brownies, I like to line my pan with parchment paper and lightly grease it to prevent any sticking.
  • Thick or thin. Use a 9×9″ baking dish for thicker brownies and a 9×13 inch baking dish for thinner brownies.
  • Extra marshmallows. If using a 9×13″ pan, I’d recommend adding an extra cup of marshmallows and doubling the frosting.
  • Chocolate lovers. Next, add 1/2 cup dark or milk chocolate chips to the brownie batter for even more chocolate flavor.
  • Slicing. Use a plastic knife to easily cut the brownies and wipe them clean after each cut. Make sure they’ve cooled for about 30 minutes before cutting.
  • Don’t overbake! The brownies should be gooey and fudgy. I use the toothpick method to check if they’re done. Stick a toothpick in the center of the brownies and it should come out clean.

Storing

  • Store: These marshmallow brownies are simple to store. Place in an airtight container and store at room temperature or in the refrigerator. Enjoy within the week!
  • Freeze: You can freeze marshmallow brownies in an airtight container for up to two months in the freezer.
Marshmallow brownies sliced in the pan.

More brownie recipes to to make include homemade cosmic brownies, chocolate mint brownies and Oreo brownies.

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stacked marshmallow brownies

Marshmallow Brownie Recipe

5 from 36 votes
Recipe by: Jamielyn Nye
Prep Time: 15 minutes
Cook Time: 30 minutes
Total Time: 45 minutes
Servings: 20
Rich and decadent marshmallow brownies made with a fudgy brownie mix then topped with gooey marshmallows and a chocolate frosting.
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Ingredients 

  • 18 ounce box brownie mix , plus ingredients on box
  • 2 cups mini marshmallows , more if needed

Chocolate frosting:

Instructions

Make the brownies:

  • Preheat the oven to 350ºF. For thick brownies, spray a 9×9" pan with non-stick spray or line with parchment paper. If you'd like to make thinner brownies, use a 9×13" pan.
  • Prepare the brownie mix according to the package directions. Then pour batter into the prepared pan. Bake for 30 minutes, or until almost done. Do not overcook. Remove from oven and then sprinkle 2 cups mini marshmallows over the top so that they are covering the brownies in an even layer. Bake an additional 3 minutes, or until the marshmallows have puffed up. Remove from oven and allow to cool.

Make the frosting:

  • Combine 1/4 cup salted butter, 1 1/2 cups powdered sugar, 2 Tablespoons cocoa powder, 2 Tablespoons milk and 1/2 teaspoon vanilla in a large bowl. Beat with electric beaters until smooth. Add more milk or powdered sugar if needed to reach a smooth and creamy frosting. Then place spoonfuls on top of the marshmallows once cooled. Gently spread evenly.
  • I like to cut the brownies in 5×4 rows with a plastic knife.

Last step: Please leave a comment and rating after you make the recipe.

Notes

If you make in a 9×13″ pan, you may want to double the frosting and use an extra cup of mini marshmallows. 
Make sure to use fresh marshmallows for optimal flavor. 
If you want to make these brownies even richer, add 1/2 cup chocolate chips in the brownie batter before baking. 

Nutrition

Calories: 185kcal, Carbohydrates: 33g, Protein: 1g, Fat: 5g, Saturated Fat: 2g, Cholesterol: 6mg, Sodium: 99mg, Potassium: 6mg, Fiber: 1g, Sugar: 24g, Vitamin A: 71IU, Calcium: 2mg, Iron: 1mg

Nutrition provided is an estimate. It will vary based on specific ingredients used.