Preheat oven to 350°F. Spray a 10-cup nonstick Bundt pan or tube pan with cooking spray.
Sift flour, cocoa powder, baking powder, baking soda, and salt into a large bowl. Stir to combine.
In another large bowl, whisk together egg, egg whites, buttermilk, and margarine. One at a time, add Splenda Brown Sugar Blend, applesauce, corn syrup, and vanilla extract, whisking well after each addition.
Add the dry ingredients gently fold in with rubber spatula just until combined. Transfer batter into prepared pan.
Bake for 40–50 minutes, or until a wooden pick inserted in center of cake comes out clean. Cool in pan 5 minutes, then remove to wire rack and cool completely.