Everyone will flip for this stunning dessert crowned with shingled plum slices. The addition of buttermilk to the batter keeps the cake underneath wonderfully moist. This recipe comes from Martha Stewart's Cake Perfection: 100+ Recipes for the Sweet Classic, From Simple to Stunning (Clarkson Potter).
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.
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.
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.
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.
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.
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.
Recipes reprinted from: Martha Stewart's Cookie Perfection: 100+ Recipes to Take Your Treats to the Next Level, From the Kitchens of Martha Stewart Living. Copyright © 2020 by Martha Stewart Living Omnimedia, Inc. Published by Clarkson Potter/Publishers, an imprint of Random House, a division of Penguin Random House LLC. Available October 13th wherever books are sold; to pre-order, go to amazon.com.