Preheat your oven to 350°F (175°C). Grease an 8×8 inch baking dish or line it with parchment paper.

In a large bowl, whisk together cocoa powder, granulated sugar, brown sugar, and flour until combined. Add eggs and vanilla; mix until just combined.

Pour the brownie batter into the prepared dish and spread evenly. Bake for 25-30 minutes until a toothpick inserted comes out with moist crumbs.

Remove from oven and immediately dollop marshmallow fluff over the warm brownies, spreading evenly.

Broil for 1-2 minutes until the marshmallow fluff is bubbly and lightly toasted—watch closely to avoid burning.

Let cool for at least 10 minutes before slicing into squares.