1. Preheat oven to 350°.
2. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 5 ingredients (through salt) in a large bowl. Combine 1/2 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, 1/2 cup chocolate, and 1/4 cup nuts to flour mixture; stir to combine.
3. Pour the batter into a 9-inch square metal baking pan coated with cooking spray; sprinkle with remaining 1/4 cup nuts. Bake at 350° 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.