Mini Grapefruit Upside-Down Cakes

Preheat oven to 350°F. Remove and discard peel and pith from grapefruit. Cut 2 (1/4-inch-thick) rounds from each grapefruit. Reserve remaining grapefruit for another use. Grease 6 (8-ounce) ramekins with butter. Sprinkle 2 teaspoons sugar onto bottom and sides of each ramekin. Press 1 grapefruit round in bottom of each ramekin, trimming to fit. Set aside.

Whisk together all-purpose flour, almond flour, baking powder, and salt in a medium bowl. Set aside.

Beat butter and remaining 3/4 cup sugar in a large bowl with an electric mixer on high speed until light and fluffy, about 4 minutes. Add eggs 1 at a time, beating well after each addition. Beat in zest. Reduce mixer speed to low; add flour mixture to butter mixture alternating with sour cream and milk in 2 additions and beating until incorporated after each addition. Spoon batter over grapefruit rounds.

Arrange ramekins evenly on a rimmed baking sheet. Bake in preheated oven until a wooden pick inserted in center of cakes comes out clean, 24 to 26 minutes. Remove from oven. Let cool in ramekins 10 minutes. Run a knife around edge of each ramekin, and invert cakes onto plates. Serve warm. Top with whipped cream, if desired.