Heat a convection oven to 350 degrees F or a conventional oven to 375 degrees F.Spray a 9-by-13-inch baking pan with olive oil spray.
Combine the sugar, flour, cornmeal, baking powder and orange zest in a stand mixer fitted with the paddle attachment on low speed.
Add the cream and milk to the egg yolks and whisk together. Pour into the dry ingredients. Add the butter, Marsala and vanilla extract, and mix on low speed until combined, scraping the sides and bottom as needed. Add the grapes and mix briefly to disperse.
Pour the batter into the prepared pan and bake until well browned on the bottom, about 20 minutes. Serve warm or at room temperature.
I use an imported Italian polenta called bramata, a word that refers to the coarse grind of the grain.