Next-Level Plum Upside-Down Cake

1. Preheat oven to 350°F. In a 10-inch cast-iron skillet, melt 4 tablespoons butter with brown sugar over low heat. Using a fork, mix together until smooth. Remove from heat.

2. Place plum halves, cut-sides down, on a work surface; cut into 1/4-inch-thick slices. Transfer each slice to skillet, forming concentric circles beginning at outer edge and ending in center, overlapping slices as you go around.

3. In the bowl of a stand mixer fitted with the paddle attachment, beat remaining 1 stick butter with granulated sugar on medium-high speed until light and fluffy. In a small bowl, beat egg yolks and vanilla; add to butter mixture. In another small bowl, whisk together flour, baking powder, and salt. With mixer on low speed, add flour mixture to butter mixture in three additions, alternating with buttermilk and beginning and ending with flour mixture; beat until just combined. Transfer batter to a wide bowl.

4. In a clean mixer bowl fitted with the whisk attachment, beat egg whites on medium-high until soft peaks form, about 2 minutes. Stir one-third of whites into batter. Gently fold in remaining whites until incorporated. Pour batter over plums, spreading with an offset spatula to cover evenly.

5. Bake until top is browned and springs back when lightly pressed in center, and a tester comes out clean, 50 minutes to 1 hour. Transfer skillet to a wire rack; let cool 20 minutes. Turn out cake onto a serving plate and let cool completely before serving.

Cook's Notes

If you don't have a 10-inch cast iron skillet, use a 9-by-2-inch round cake pan. When forming concentric circles of plum slices in the pan, shingle them as closely as possible—they'll shrink while they bake.