Step 1: Prepare Your Pan
Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal. Lightly grease the parchment with butter or cooking spray.

Step 2: Make the Brownie Batter
In a medium saucepan over low heat, melt the butter completely. Remove from heat and stir in the sugar until well combined. The mixture will look grainy—this is normal. Add the eggs one at a time, whisking well after each addition. Stir in the vanilla extract.

Step 3: Add Dry Ingredients
Sift the cocoa powder, flour, salt, and baking powder directly into the wet ingredients. Fold gently with a rubber spatula until just combined. Don’t overmix—a few small streaks of flour are okay. Overmixing creates tough, cakey brownies instead of fudgy ones.

Step 4: Bake the Brownies
Pour the batter into your prepared pan and spread evenly with a spatula. Bake for 25-28 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs (not wet batter). The brownies will continue cooking as they cool, so slightly underbaking ensures fudgy texture.

Step 5: Add the Marshmallow Layer
Remove the brownies from the oven and immediately sprinkle the mini marshmallows evenly over the hot surface. Return to the oven for 2-3 minutes, just until the marshmallows puff up and begin to melt. Don’t leave them too long or they’ll turn golden brown (unless you prefer that look!). Remove and let cool while you make the frosting.

Step 6: Prepare the Chocolate Frosting
In a medium saucepan, melt the butter over low heat. Whisk in the cocoa powder until smooth. Remove from heat and add the milk, stirring to combine. Gradually add the powdered sugar, one cup at a time, whisking until smooth and glossy. Stir in the vanilla extract.

Step 7: Frost and Cool
While the marshmallow layer is still slightly warm and sticky, pour the frosting over the top. Use an offset spatula to spread it evenly, working quickly before the frosting sets. The warmth helps the frosting spread smoothly and creates a beautiful glossy finish. Let the brownies cool completely at room temperature (about 2 hours) or refrigerate for 30-45 minutes to speed up the process.

Step 8: Cut and Serve
Once completely cool, use the parchment overhang to lift the entire brownie slab out of the pan. Place on a cutting board and use a sharp knife (wiped clean between cuts) to slice into 24 squares. For cleanest cuts, chill the brownies before slicing.