In a bowl, beat eggs on high for 3 minutes; gradually add sugar. Combine flour and baking powder; stir into egg mixture alternately with water. Beat on low for 1 minute. Pour into a greased 9-in. square baking pan. Bake at 375° for 25-30 minutes or until the cake tests done. Cool. Split cake into two layers.
For filling, combine the sugar and cornstarch in a saucepan; stir in milk. Bring to a boil; cook for 2 minutes. Stir a small amount into eggs; mix well. Return all to pan. Cook and stir for 1 minute or until thick. Remove from the heat; cool slightly. Add the butter and vanilla; cool completely. Spread between cake layers. Cut into squares; top with pie filling and whipped cream. Yield: 9 servings.