Combine butter and chocolate chips in a small saucepan over low heat, stirring frequently until melted.
Remove from heat; stir in milk and vanilla.
Whisk together flour, cocoa, baking powder, and salt in a large bowl; stir into chocolate.
Combine sugar and egg in a bowl and stir into chocolate mixture. Spoon batter into muffin cups.
Bake 7 minutes. Top each brownie with 1 teaspoon coconut or walnuts, if desired; bake 5 more minutes. Cool on rack. If you didn't top with coconut or walnuts, sprinkle with powdered sugar.