Allow egg whites and butter to stand at room temperature for 30 minutes. Meanwhile, grease and lightly flour two 9-inch or 8-inch round cake pans or line with parchment paper; set aside. Stir together flour, baking powder, and salt in a medium bowl; set aside.
Preheat oven to 375°F. Beat vegetable oil spread and the butter in a large bowl with an electric mixer on medium to high speed for 30 seconds. Gradually add sugar, about 2 tablespoons at time, beating on medium speed until well mixed, scraping side of the bowl constantly. Beat on medium speed for 2 minutes more. Gradually add the egg whites, beating well. Beat in vanilla. Alternately add the flour mixture and milk to the butter mixture, beating on low speed after each addition just until combined. Spread the batter into the prepared pans.
Bake for 12 to 14 minutes or until a toothpick inserted near the centers comes out clean. Cool the cake layers in the pans on wire racks for 10 minutes. Remove the cake layers from the pans. Cool completely on wire racks.
Tip: If using a sugar substitute, we recommend Splenda Sugar Blend for Baking. Follow package directions to use product amount equivalent to ¾ cup sugar. Nutrition Per Serving with Substitute: Same as below, except 160 cal., 22 g carb. Exchanges: 1½ other carb. Carb choices: 1½.