Preheat oven to 350°F. Spray pan with vegetable pan spray.
In medium bowl, combine flour, cocoa, baking powder, baking soda and salt. In large bowl, beat butter, brown sugar and granulated sugar with electric mixer until light and fluffy. Add melted chocolate, milk, vanilla and eggs, one at a time; beat well. Gradually add flour mixture, mixing well after each addition. Pour into prepared baking pan.
Bake 35-40 minutes or until cake tester inserted in center comes out clean. Cool in pan on cooling grid 10 minutes. Remove from pan and cool completely.
Makes about 6 cups batter; about 12 servings.