- 1/2 cup (1 stick) plus 1 tablespoon unsalted butter, plus extra for the pan, at room temperature
- 4 firm, ripe plums, each cut into 8 wedges
- 1/4 cup plus 2/3 cup granulated sugar
- 1 cup all-purpose flour
- 3/4 teaspoon baking powder
- 1/4 teaspoon baking soda
- 1/4 teaspoon kosher salt
- 1 large egg
- 2/3 cup sour cream
- 1 teaspoon pure vanilla extract
- Heat oven to 350° F. Butter an 8-inch cake pan and line the bottom with parchment paper.
- Melt 1 tablespoon of the butter in a large skillet over medium-high heat. Add the plums and 1/4 cup of the sugar and cook, tossing, until the sugar dissolves and the juices from the plums become syrupy, 3 to 4 minutes. Arrange the plums in the cake pan in slightly overlapping concentric circles, starting from the outside. Spoon any pan juices over the top.
- In a small bowl, whisk together the flour, baking powder, baking soda, and salt.
- With an electric mixer, beat the remaining 1/2 cup of the butter and 2/3 cup of the sugar until fluffy. Beat in the egg, sour cream, and vanilla. Gradually add the flour mixture, mixing just until incorporated.
- Pour the batter over the plums and bake until a toothpick inserted in the center comes out clean, 50 to 55 minutes. Let cool in the pan for 1 hour. Place a large plate over the cake pan and invert the cake onto the plate.