Mississippi mud pie brownies take the classic Southern dessert—a dense, fudgy brownie base topped with a gooey marshmallow layer and a glossy chocolate frosting—and turn it into an easy, shareable bar. The combination of a slightly crisp top, a moist center, and a creamy, almost truffle-like frosting makes this a crowd-pleaser that tastes far more complicated than it is. Using a boxed brownie mix as the base keeps the process simple, while the homemade frosting and marshmallow layer give it that authentic, from-scratch feel.
Ingredients
- 1 box (18.3 oz / 520 g) fudge brownie mix (plus ingredients listed on the box: usually 1/2 cup vegetable oil, 2 large eggs, and 1/4 cup water)
- 2 cups (120 g) mini marshmallows
- 1/2 cup (115 g) unsalted butter
- 1/4 cup (60 ml) whole milk
- 1/4 cup (25 g) unsweetened cocoa powder
- 3 cups (360 g) powdered sugar, sifted
- 1 teaspoon vanilla extract
- Pinch of salt
- Optional: 1/2 cup (85 g) semisweet chocolate chips, for extra richness
Instructions
- Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving some overhang on two opposite sides for easy lifting.
- Prepare the brownie mix according to the package directions (usually mixing the mix, oil, eggs, and water until just combined). Pour the batter into the prepared pan and spread it evenly.
- Bake the brownies for 25–30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter). The brownies should be just set—they will continue to cook slightly as they cool.
- Remove the pan from the oven and immediately sprinkle the mini marshmallows evenly over the hot brownies. Return the pan to the oven for 2–3 minutes, just until the marshmallows start to puff and soften. Do not let them brown completely; you want them gooey, not toasted.
- While the marshmallows are in the oven, make the frosting. In a medium saucepan over medium heat, melt the butter. Stir in the milk, cocoa powder, and salt, and whisk until smooth and slightly bubbling, about 2 minutes.
- Remove the saucepan from the heat. Gradually whisk in the powdered sugar, about 1 cup (120 g) at a time, until the frosting is thick and glossy. Stir in the vanilla extract. If the frosting seems too thick, add a teaspoon of milk at a time; if too thin, add a little more powdered sugar.
- Pour the warm frosting over the marshmallow layer, spreading it gently to cover completely. If using chocolate chips, sprinkle them over the top while the frosting is still warm.
- Let the brownies cool completely at room temperature, about 30–40 minutes, until the frosting is set. For cleaner squares, refrigerate for 1 hour before slicing.
- Lift the brownies out of the pan using the parchment overhang, place on a cutting board, and slice into 16 squares with a sharp knife. Wipe the knife clean between cuts for neat edges.
Tips
- Don’t overbake the brownies. They should be slightly underdone when you add the marshmallows, because they’ll finish baking during the extra 2–3 minutes. Overbaked brownies will be dry and crumbly.
- Use mini marshmallows. Regular-sized marshmallows can be too bulky and won’t melt as evenly. If you only have large ones, cut them in half with kitchen scissors.
- For a thicker, more decadent brownie layer, use a 9×9-inch pan instead of 9×13-inch. You’ll need to increase the baking time by about 5–8 minutes, and the bars will be taller.
- Substitute the boxed mix with a homemade brownie recipe if you prefer—just make sure it yields about 6 cups (1.4 L) of batter and bakes in a 9×13-inch pan.
Serving Suggestions
Serve these brownies slightly warm with a scoop of vanilla ice cream and a drizzle of chocolate syrup. For a Southern-style dessert, pair them with sweet tea or coffee. They’re also delicious crumbled over a bowl of vanilla pudding or used as a base for an ice cream sundae.
Variations
- Peanut Butter Mud Pie: Swirl 1/2 cup (130 g) creamy peanut butter into the brownie batter before baking, and add 1/2 cup (75 g) chopped peanuts to the frosting.
- Salted Caramel: Drizzle 1/2 cup (160 g) caramel sauce over the marshmallow layer before adding the frosting, and sprinkle flaky sea salt on top.
- Mint Chocolate: Add 1/2 teaspoon peppermint extract to the frosting and top with crushed chocolate mints.
- Gluten-Free: Use a gluten-free brownie mix and ensure your marshmallows and cocoa powder are certified gluten-free.
Storage and Reheating
Store the brownies in an airtight container at room temperature for up to 3 days, or in the refrigerator for up to 5 days. For longer storage, wrap individual squares in plastic wrap and freeze for up to 3 months. To reheat, microwave a single brownie for 15–20 seconds until warm and the frosting is slightly gooey. If frozen, thaw in the refrigerator overnight before reheating.
Frequently Asked Questions
Can I use a homemade brownie recipe instead of a boxed mix? Yes, absolutely. Use your favorite brownie recipe that yields a 9×13-inch pan. Just make sure it’s fudgy, not cakey, to keep the texture authentic.
Why is my frosting grainy? Grainy frosting usually happens when the powdered sugar isn’t sifted or when the butter/milk mixture is too hot. Sift the sugar and let the butter mixture cool for a minute before whisking in the sugar.
Can I make these ahead of time? Yes, you can make them a day in advance. Store them covered in the refrigerator, and let them sit at room temperature for 30 minutes before serving.
How do I get clean, neat squares? Chill the brownies completely in the refrigerator for at least 1 hour before slicing. Use a large, sharp knife and wipe it clean with a damp paper towel between cuts.
My marshmallows melted into the brownies completely—is that okay? That’s fine! It means they were baked a bit too long, but the result will still be delicious. The marshmallow layer will be more like a chewy caramel. Next time, watch them closely and remove them as soon as they puff.
Can I freeze these brownies? Yes, they freeze beautifully. Wrap them tightly in plastic wrap and then foil, and freeze for up to 3 months. Thaw in the refrigerator before serving.
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