Red-Grape Cake with Whipped Creme Fraiche

Preheat oven to 350°F. Butter a 9-inch round cake pan and line bottom with parchment; butter parchment. In a medium bowl, whisk together flour, baking powder, and salt.

In a large bowl, beat together butter and granulated sugar on medium-high speed until light and fluffy, about 2 minutes. Add eggs, one at a time, beating well after each addition. Fold in half of flour mixture, then sour cream and lemon zest. Stir in remaining flour mixture just to combine. Spread batter evenly into prepared pan, smoothing top with a small offset spatula. Scatter grapes over top.

Bake until cake pulls away from sides of pan and a tester comes out clean, 45 to 50 minutes. Transfer to a wire rack; let cool 20 minutes. Turn cake out onto rack, remove parchment, invert cake, and let cool completely.

In a large bowl, beat cream and confectioners' sugar on medium-high speed to soft peaks, about 3 minutes. Add crème fraîche; beat to firm peaks, about 2 minutes more. Serve cake with whipped crème fraîche and more grapes.