Preheat oven to 350°F.
Combine flour, sugar, cocoa powder, brown sugar, baking powder and salt in a large bowl. Combine ½ cup chocolate and milk in a microwave-safe bowl; microwave at HIGH 1 minute, stirring after 30 seconds. Stir in butter, vanilla and eggs. Add milk mixture, ½ cup chocolate and ¼ cup nuts to flour mixture; stir to combine.
Pour the batter into a 9-inch square metal baking pan coated with cooking spray; sprinkle with remaining ¼ cup nuts. Bake at 350°F for 19 minutes or until a wooden pick inserted in center comes out with moist crumbs clinging. Cool in the pan on a rack. Cut into squares.
