1 cup all-purpose flour
1/2 teaspoon baking powder
1/2 teaspoon baking soda
1/4 teaspoon salt
1/2 stick unsalted butter, softened
2/3 cup plus 1 1/2 tablespoons sugar, divided
1/2 teaspoon pure vanilla extract
1 large egg
1/2 cup well-shaken buttermilk
Plums and nectarines, chopped (I used a mix of the 2 fruits-to equal 1 1/4 cups)
Turbinado sugar (I added this)
Preheat oven to 400°F with rack in middle. Butter and flour a 9-inch round cake pan.
Whisk together flour, baking powder, baking soda, and salt.
Beat butter and 2/3 cup sugar with an electric mixer at medium-high speed until pale and fluffy, about 2 minutes, then beat in vanilla. Add egg and beat well.
At low speed, mix in flour mixture in 3 batches, alternating with buttermilk, beginning and ending with flour, and mixing until just combined.
Spoon batter into cake pan, smoothing top. Scatter chopped plums and nectarines evenly over top and sprinkle with remaining 1 1/2 Tbsp sugar. I also used a little Turbinado sugar. Just sprinkle some over the cake if you wish. It gave it a nice extra sugary crunch.
Bake until cake is golden and a wooden pick inserted into center comes out clean, 25 to 30 minutes. Cool in pan 10 minutes, then turn out onto a rack and cool to warm, 10 to 15 minutes more. Invert onto a plate.