Preheat oven to 350˚F/175°C. Grease and flour a 9x13-inch pan. In a medium saucepan, melt together the butter and cocoa. Cool slightly. Mix flour and sugar together. Add the chocolate mixture to the dry ingredients. Mix well and add eggs, one at a time, blending well after each egg until batter is smooth. Fold in vanilla and chopped bananas.
Pour half of the batter into prepared pan. Gently push peanut butter cups into the batter. Add the remaining batter and smooth to cover the peanut butter cups. Bake for 30-35 minutes. Do not over-bake, brownies can be slightly sticky to the touch.
Quickly make the frosting. In a medium saucepan, combine the butter, cocoa, and milk. Heat to boiling while stirring. Remove from heat and whisk in powdered sugar and vanilla until frosting is smooth. Pour over hot brownies. Sprinkle with chopped peanuts. Cool and cut into 24 brownies.