Preheat oven to 350°. Generously butter mini mu∞n tins; dust with cocoa, tapping out excess. Whisk together flour, cocoa, baking powder, and salt in a bowl.

Melt butter and chopped chocolate in a heatproof bowl set over (not in) a saucepan of gently simmering water; remove bowl from pan. Add sugar; stir to combine.

Add eggs; mix to combine. Stir in flour mixture just until combined (do not overmix).

Spoon batter into prepared tins, filling each cup just over halfway. Bake until a cake tester comes out with only a few crumbs attached, about 10 minutes.

Remove from oven, and sprinkle evenly with chocolate chips, peanuts, and marshmallows.

Continue baking until chocolate is shiny and marshmallows are puffed and browned in spots, about 7 minutes. Transfer tins to wire racks; let cool completely. (Brownies can be stored in an airtight container at room temperature up to 3 days.)