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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.
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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.
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Add eggs; mix to combine. Stir in flour mixture just until combined (do not overmix).
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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.
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Remove from oven, and sprinkle evenly with chocolate chips, peanuts, and marshmallows.
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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.)