Spray a 9x1-1/2-inch round baking pan with nonstick spray; set pan aside.
In a medium mixing bowl combine flour, sugar, cocoa powder, and baking soda. Add water, cooking oil, vinegar, and vanilla. Stir until thoroughly combined.
In a medium mixing bowl beat the egg whites with an electric mixer on high speed until stiff peaks form (tips stand straight). Fold into batter. Pour batter into the prepared baking pan.
Bake in a 350 degree F oven about 25 minutes or until wooden toothpick inserted near the center comes out clean. Cool cake in pan on wire rack for 10 minutes. Remove cake from pan and cool thoroughly on wire rack. Spread whipped desert topping over top and sides of cooled cake. Top with pie filling. Makes 8 servings.