Place a rack in the lower third of the oven and preheat to 350. In a large bowl, whisk together the flour, sugar, cornmeal, baking powder, and salt. Make a well in the center; set aside.

In a 9 1/2-inch cast-iron skillet melt the butter over medium heat. Pour 6 tablespoons of the melted butter into a heatproof cup (to reserve for the batter). Add the brown sugar to the remaining butter in the skillet and cook until melted, about 1 minute. Stir in the grapes and remove from the heat.

In small bowl, beat the milk, eggs, and vanilla together, and then stir with a whisk into the dry mixture, just until smooth. Pour in the reserved melted butter and the olive oil; stir until blended. Carefully pour the batter over the warm grape mixture in the skillet, smoothing the top. Bake until cake is firm and springy in the center, about 35 minutes.

Let the cake cool in the pan for 5 minutes. Invert a cake platter on top of the skillet and then carefully flip and remove the skillet. Let it cool for at least 20 minutes before serving.