Beat peanut butter, cream cheese, and margarine at medium speed of an electric mixer until creamy; gradually add sugar, beating well. Add egg substitute, beating well.
Combine flour and next 3 ingredients; stir well with a wire whisk. Add to peanut butter mixture alternately with buttermilk, beginning and ending with flour mixture. Mix batter just until blended after each addition. Stir in vanilla.
Pour batter into a 13- x 9- x 2-inch pan coated with cooking spray. Bake at 350° for 30 to 35 minutes or until a wooden pick inserted in center comes out clean. Cool completely on a wire rack. Spread Chocolate Frosting over cooled cake, and sprinkle with chopped peanuts.