Preheat oven to 350°.
To prepare cake, coat bottom of a 13 x 9-inch baking pan with cooking spray (do not coat sides of pan); line bottom of pan with wax paper. Coat wax paper with cooking spray; dust with 1 tablespoon flour. Set aside.
Combine 1/2 cup butter and sour cream in a large bowl, stirring with a whisk until well blended. Add granulated sugar and 2 teaspoons vanilla. Beat with a mixer at medium speed 3 minutes or until well blended. Add egg substitute; beat 2 minutes or until well blended.
Lightly spoon 2 cups flour into dry measuring cups; level with a knife. Combine 2 cups flour, baking soda, and 1/2 teaspoon salt, stirring 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 at 350° 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.
To prepare frosting, place the cream cheese, 1/4 cup butter, and 1 teaspoon vanilla in a large bowl; beat with a mixer 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. Gradually add remaining 1 1/2 cups powdered sugar. Place cake on a serving platter. Spread frosting over top and sides of cake. Store cake loosely covered in the refrigerator.