Step 1: Preparing the Brownie Base

1. Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking pan or line it with parchment paper.
2. In a heatproof bowl, melt butter and semi-sweet chocolate chips over a double boiler or microwave in short bursts, stirring until smooth.
3. In another bowl, whisk together granulated sugar, brown sugar, eggs, and vanilla extract until light and creamy.
4. Slowly add the melted chocolate mixture to the sugar-egg mixture, stirring continuously.
5. Sift in flour, cocoa powder, baking powder, and salt and mix until just combined—do not overmix.
6. Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

Step 2: Adding the Marshmallows and Nuts

7. Once the brownies come out of the oven, immediately sprinkle mini marshmallows evenly over the hot surface.
8. Add chopped walnuts or pecans, gently pressing them into the melted marshmallows.

Step 3: Creating the Chocolate Glaze

9. In a small saucepan, heat heavy cream until it just begins to steam (but don’t let it boil).
10. Pour the hot cream over chopped dark chocolate in a bowl and let it sit for a minute, then stir until smooth.
11. Drizzle the warm glaze over the marshmallow-nut topping, spreading evenly with a spatula.
12. Let the brownies cool at room temperature or in the refrigerator for at least 30 minutes before slicing.