Ingredients
Equipment
Method
Bake the brownie base
- Preheat oven to 350F and grease a 9x13 pan. Set the pan aside for immediate batter baking.
- Melt the unsalted butter and whisk in the sugar, eggs, and vanilla until smooth and glossy. Scrape the sides so the mixture is evenly combined.
- Fold in the flour, cocoa, and salt just until no dry streaks remain. The batter should look thick and chocolate-colored.
- Spread the batter in the prepared 9x13 pan and bake for 25-28 minutes until set. Look for a firm surface with a fudgy center that holds its shape.
Add the marshmallow layer
- Immediately scatter the mini marshmallows over the hot brownies in an even layer. The marshmallows should begin melting at the edges right away.
- Return to the oven for 3-4 minutes until the marshmallows puff and turn golden. Watch closely so they don’t brown too far.
Make and pour the chocolate frosting
- Melt the unsalted butter and whisk in the cocoa powder and whole milk until smooth. Keep whisking until the mixture looks uniformly dark.
- Remove from heat and whisk in powdered sugar and vanilla until thick and pourable. It should coat the whisk with a ribbon-like texture.
- Pour the warm frosting over the marshmallow layer, covering the surface completely. Let it fall in an even sheet so it levels out.
- Scatter walnuts or pecans over the frosting while it’s still warm and slightly soft. Press very gently so pieces adhere.
Chill and slice
- Refrigerate for 2 hours before cutting. The frosting should set firmly so slices come out clean.
Notes
For the cleanest slices, chill uncovered for the first 30 minutes, then cover once fully set to prevent sticking. Store covered in the refrigerator up to 4 days; freeze slices in a freezer-safe container up to 2 months and thaw in the fridge. To make it less sweet, use unsweetened cocoa for the frosting and reduce powdered sugar by 1/2 cup while whisking for thickness.