1Preheat oven to 350^. Line bottom of 2-9" round cake pans with parchment; coat parchment and sides of pans with nonstick cooking spray; set aside.
For the sugar topping, in a small bowl combine sugar and 1 tsp. of the lemon zest, pressing and stirring until well blended. Set aside.
2In a large bowl and using electric mixer, prepare cake mix according to package directions. Stir in remaining 2 tsp. lemon zest; mix until well combined. Fold in 1 cup of the grapes. Pour batter evenly into cake pans.
3Bake 20 minutes. Remove cake from oven; place remaining 1-1/2 cups grapes evenly on tops of cakes. Sprinkle evenly with sugar topping.
Bake 10 to 15 minutes more or until a toothpick inserted into cake comes out clean. Remove from oven; let cool in pans 10 minutes. Carefully transfer cakes, sugar side up, onto serving platters. Let cool 20 minutes before serving.