Tip:If you add all the grapes at once, they’ll sink to the bottom, so reserve half and scatter them on top of the cake after it’s been in the oven for 10 minutes. They’ll sink slightly but still be visible. Dust the cake with confectioners’ sugar before serving for a pretty presentation.
Preheat the oven to 350ºF. Grease a 9-inch round springform pan. Whisk together the flour, cornmeal, baking powder, and salt in a medium bowl.
Combine the eggs and sugar in a large mixing bowl. Beat on medium-high speed until light in color and increased in volume, about 5 minutes. With the mixer on low speed, add the oil in a slow, steady stream. Turn the mixer to medium speed and beat for 1 minute. Stir in the milk, vanilla, and lemon zest on low speed.
With the mixer on low speed, add the flour mixture, 1⁄2 cup at a time, until just incorporated. Stir in 1⁄2 of the grapes. Scrape the batter into the prepared pan and bake for 10 minutes.
Scatter the remaining grapes over the top of the partially baked cake and continue to bake until the cake is golden and a toothpick inserted in the center of the cake comes out clean, about 40 minutes longer.
Transfer the pan to a wire rack and let cool for 5 minutes. Release the sides from the pan and let the cake cool completely before dusting with confectioners’ sugar, cutting into wedges, and serving.
Store uneaten cake in a cake keeper or wrap in plastic and store at room temperature for up to 3 days.
If it is a dinner party dessert, serve it with an Italian sweet wine like Vin Santo or with small glasses of grappa.