Prep: Preheat the oven to 350°F. Line a 9x9" pan with parchment paper and set aside.
Chocolate mixture: In a small mixing bowl, add the chopped chocolate and cocoa powder. Set aside.
Brown the butter: In a small pan, heat the butter on medium heat, stirring often, until the butter has melted and the white milk solids start separating out. The butter will start sizzling. When the sizzling slows down, the surface starts getting foamy, and the milk solids sink to the bottom of the pan, stir it constantly and watch for the milk solids to turn golden brown. Remove it from the heat the moment they have browned to prevent them from burning.
Melt chocolate: Pour the browned butter into your bowl of chocolate and cocoa powder. You should have 125g of browned butter. If you have less than that, top it off with a bit more butter to make it 125g. Let the mixture sit undisturbed for about 2 minutes before stirring it until all the chocolate has melted and the mixture is smooth. Set aside.
Whip eggs: In a large mixing bowl, add the eggs, granulated sugar, and brown sugar. Use an electric hand mixer to beat together on medium-high speed for 5 full minutes until it's pale in colour and doubled in volume.
Combine: With the mixer on low speed, slowly pour in the melted chocolate mixture. Mix until combined. Add the flour and use a spatula to fold together until just combined.
Fill pan: Transfer the batter to your lined pan. Use an offset spatula to smooth the batter into a flat layer.
Bake: Bake brownies for 23-25 minutes or until the edges are set and the middle is just slightly jiggly. Remove the pan from the oven and scatter the mini marshmallows on top. Return it to the oven to bake for another 1-2 minutes just until the marshmallows have puffed up slightly and the heat helps them stick.
Cool and slice: Let the brownies cool completely in the pan before slicing into squares. Dust the top with a bit of extra cocoa powder (optional). When slicing, rub a knife with a bit of cooking oil before each cut to prevent the marshmallows from stick to the knife.