This very tender, flavorful cake features juicy baked grapes and a splash of olive oil. Serve it as a breakfast, brunch, or tea cake, or top it with a dollop of sweetened whipped cream for a sophisticated dessert.
Preheat oven to 350 degrees. Butter a 9-inch metal cake pan and line the bottom with a round of parchment paper. Butter the paper and then coat the pan with flour.
In the bowl of an electric mixer, cream together butter and 2/3 cup sugar until light and fluffy, stopping to scrape down the sides of the bowl frequently. Beat in eggs 1 at a time. Beat in yolk, followed by olive oil. Sift together flour, baking powder, and salt in separate bowl. On low speed, beat in the flour mixture in three additions, alternating with additions of buttermilk and scraping down the sides of the bowl frequently. Stir in half the grapes and pour the batter into the prepared pan. Sprinkle the top with remaining grapes and remaining 1 tablespoon sugar. Bake until the cake is browned on top and springs back when pressed lightly with your finger, 50 to 60 minutes. Cool the cake in the pan 15 minutes.
Run a knife around the edges of the pan, then invert, peel off the paper, and cool the cake upright on a rack. Serve warm or room temperature.