Preheat oven to 350°.
Coat bottom of a 13 x 9-inch baking pan with cooking spray.
To prepare batter, place butter in a large bowl; beat with a mixer at medium speed until fluffy. Add sugar and 1 teaspoon vanilla; beat until well blended (about 5 minutes). Add egg whites and egg, 1 at a time, beating well after each addition. Add cocoa and fat-free milk; beat well (mixture will appear curdled). Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, and salt; stir with a whisk. Add to cocoa mixture; beat at low speed just until blended. Spoon batter into prepared pan.
To prepare topping, place cream cheese in a large bowl; beat with a mixer at medium speed until smooth. Gradually add cornstarch and remaining ingredients; beat until smooth. Spread evenly over batter. Bake at 350° for 35 minutes or until set. Cool in pan on a wire rack.