In a small bowl combine flour, cornmeal, baking powder, and salt; set aside.
In a large mixing bowl, beat the butter with an electric mixer on medium to high speed for 30 seconds. Add granulated sugar and beat until light. Add egg yolks and eggs, 1 at a time, beating after each addition. Add lemon peel and vanilla; beat until combined. Beat in the flour mixture. Spoon batter over plums in pan and spread evenly.
Bake about 50 minutes or until a wooden toothpick inserted near center comes out clean. Cool cake in pan on a wire rack for 20 minutes. Remove sides of pan; cool cake completely. Invert cake onto serving platter; remove bottom of pan and parchment. If desired, serve with whipped cream. Makes 10 servings.