1. Preheat oven to 350º.
2. Line a 12-cup minimuffin pan with paper liners. Combine butter and chocolate chips in a small saucepan over low heat, stirring frequently until melted. Remove from heat; stir in milk and vanilla.
3. 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.
4. 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.