Preheat oven to 350. Brush an 8 inch square pan with olive oil and dust with flour, tap out excess. Whisk together flour, almonds, polenta, baking powder, 1/2 teaspoon salt. Beat eggs, sugar and zest in a mixer on high until pale and fluffy. Reduce speed to low and slowly add 1/2 cup olive oil. Beat in flour mixture in 3 additions, alternating with milk, beginning and ending with flour. Spoon batter into oiled pan, scatter 1 cup grapes over top. Bake for 15 minutes, scatter remaining 1 cup grapes over cake. Bake until cake is golden and it passes the toothpick test (a toothpick inserted in the cake comes out clean), about 25 minutes. Let cool in pan on a wire rack for 15 minutes. Turn out, cut into squares, enjoy!