Preheat the oven to 350 degrees. Generously butter and flour a 9-inch springform pan, tapping out any excess flour. Set aside.

In the bowl of an electric mixer fitted with the whisk attachment, beat eggs and sugar until thick and lemon colored, about 3 minutes. Add melted butter, the olive oil, milk, and vanilla extract, and mix until blended.

Sift flour, baking powder, and salt into a large bowl. Add lemon and orange zests, and toss to coat the zest with flour. Spoon zest mixture into batter, and stir with a wooden spoon until thoroughly blended. Scrape down the sides of the bowl, and mix once more. Set aside for 10 minutes, to allow the flour to absorb the liquids.

Stir about 3/4 of the grapes into the batter. Spoon the batter into the prepared cake pan, and smooth out the top with a spatula.

Place the pan in the center of the oven. Bake for 15 minutes, then sprinkle top of the cake with remaining grapes. Bake until the top is a deep golden brown and the cake feels quite firm when pressed with a fingertip, about 40 minutes more, for a total baking time of 55 minutes. Remove to a rack to cool. After 10 minutes, run a knife along the sides of the pan. Release, and remove the side of the springform pan, leaving the cake on the pan base. Sprinkle with confectioners' sugar just before serving. Serve at room temperature, cut into thin wedges.