½cup(1 stick) unsalted butterat room temperature, plus more for buttering the pan
2 ½cupsred seedless grapes
Preheat oven to 350°F. Butter a 9-inch metal cake pan that is 2-inches deep, and line the bottom with a round of parchment paper. Butter the paper.
In a medium bowl, stir together the flour, baking powder and salt.
In another medium bowl use an electric mixer to cream together the butter and sugar until light and fluffy, scraping down the sides of the bowl at least once. Beat in the eggs one at a time. On low speed, beat in the flour mixture in three additions, alternating with the buttermilk and scraping down the sides of the bowl on occasion. Stir in 1 ½ cups of the grapes. Pour the batter into the prepared pan.
Distribute the rest of the grapes over the batter. Bake until the cake is golden brown on top and springs back when pressed lightly with your finger, about 45 minutes.
Cool the cake in the pan for 10 minutes, then run a knife around the edges of the pan, invert the pan and remove the cake. Peel off the parchment paper and cool the cake upright on a wire rack. Serve warm or at room temperature.