In a large saucepan, melt butter; remove from the heat. Stir in sugar and cocoa until smooth. Add the egg white, milk and vanilla; stir just until blended. Combine flour, baking powder and soda; stir into chocolate mixture just until blended. Stir in nuts.
Pour into a 9-in. square baking pan coated with cooking spray. Bake at 350° for 16-18 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack. Dust with confectioners' sugar.
In a small bowl, combine topping ingredients until smooth. Drizzle over brownies.