1. Preheat oven to 350°.
2. Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, cocoa, and salt in a medium bowl; stir with a whisk.
3. Place butter and chocolate in a medium microwave-safe bowl, and microwave at HIGH for 45 seconds, stirring every 15 seconds. Stir until smooth, and set aside. Cool slightly. Add 1 cup sugar, milk, 1 teaspoon vanilla extract, egg yolks, and egg; stir with a whisk to combine. Add butter mixture to flour mixture, stirring just until combined. Pour batter into an 8-inch square metal baking pan coated with cooking spray. Bake at 350° for 20 minutes or until a wooden pick inserted in center comes out almost clean.