Step 1

Preheat the oven to 350°F. Generously butter a 9-inch springform pan. Place about 2 tablespoons of flour in the pan. Turn and shake the pan so that the flour covers all greased surfaces. Turn the pan upside down over the sink and tap lightly with your hand to remove excess flour. (If you don't have a springform pan, it's fine to use a 9-inch nonstick cake pan. For extra insurance that the cake will release, butter the pan, then place a 9-inch round of parchment paper on the bottom, butter the paper, then flour the whole pan as directed above.)

Step 2

In a small bowl, whisk together the flour, salt and baking powder. Set aside.

Step 3

In the bowl of an electric mixer, beat the eggs with ⅔ cup of the sugar until pale and slightly thickened, about a minute. Add the milk, olive oil, melted butter, lemon zest, orange zest and vanilla and beat until well combined. Add the dry ingredients and mix on low speed until just combined. Using a rubber spatula, stir in 1½ cups of the grapes. Transfer the batter to the prepared pan and smooth the top with the spatula. Scatter the remaining grapes evenly over the top of the batter. Sprinkle the remaining 1 tablespoon of sugar evenly over top. Bake for about 50 minutes, or until the cake is lightly golden and a tester comes out clean. Cool the cake for about 15 minutes on a rack.

Step 4

Slide a butter knife around the inside edge of the pan to loosen the cake. Unfasten the latch on the side of the pan and gently ease the side of the pan off of the cake. Slide a wide metal spatula between the bottom of the cake and the pan, lift the cake off the base and transfer to a serving plate. (If you used a regular nonstick cake pan, simply invert the cake onto a plate and flip over.)

Step 5

Freezer-Friendly Instructions: The cake can be frozen for up to 3 months. After it is completely cooled, wrap it securely in aluminum foil, freezer wrap or place a freezer bag. Thaw overnight on the countertop before serving.