Coat bottom only of a 13 x 9-inch baking pan with cooking spray; line bottom of pan with wax paper. Coat wax paper with cooking spray; dust with 1 tablespoon flour. Set aside. (If you plan to serve the cake straight from the pan instead of unmolding it, you can skip the wax paper step.).
Cake: Stir 1/2 cup butter and sour cream with a whisk until well blended.
Add granulated sugar and 2 teaspoons vanilla; beat at medium speed 3 minutes or until well blended.
Add egg substitute; beat 2 minutes or until well blended.
Stir 2 cups flour, baking soda, and 1/2 teaspoon salt well with a whisk.
Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture; mix after each addition.
Pour batter into prepared pan. Sharply tap pan once on counter to remove air bubbles.
Bake for 30 minutes or until a wooden pick inserted in center comes out clean.
Cool in pan 10 minutes on a wire rack; remove from pan.
Carefully peel off wax paper; cool completely on wire rack.
Frosting: Beat the cream cheese, 1/4 cup butter, and 1 teaspoon vanilla at high speed until fluffy.
Add cocoa, milk, and 1/8 teaspoon salt; beat at low speed until well blended.
Gradually add 1 1/2 cups powdered sugar; beat at low speed until creamy.