Heat oven to 350ºF. Butter an 8-inch square pan and line with a strip of parchment overhanging on two sides. Butter parchment; set aside.
In a saucepan, melt butter, chocolate and cocoa on low heat, stirring until smooth. Remove from heat and stir in sugar, eggs, flour, salt, nuts, milk and vanilla until combined.
Spread batter into prepared pan. Bake until just set, 20 min. Remove from oven, top with marshmallows and bake until marshmallows are puffed, another 5 min. Immediately sprinkle with peppermint. Cool in pan. Loosen edges and lift brownies from pan using parchment. Use a buttered knife to cut into squares. Store airtight up to 1 day.
Before slicing these brownies with marshmallows, grease your knife to keep it from sticking to the fluffy white puffs!