Step 1

Preheat the oven to 375°F (191°C). Butter an 8- or 9- or 10-inch ovenproof skillet (preferably cast-iron) and dust it with flour.

Step 2

In a bowl, whisk together the flour, baking powder, baking soda, and salt. With an electric mixer on medium, beat the butter and 3/4 cup sugar until pale and fluffy, 3 to 5 minutes.

Step 3

Beat in the egg until combined. Reduce the speed to low and add the flour mixture in 3 batches, alternating with the buttermilk in 2 additions, and mix until combined.

Step 4

Scrape the batter into the prepared skillet and smooth the top. Arrange the plums on top, fanning the slices. Sprinkle with the remaining 2 tablespoons sugar.

Step 5

Place the skillet on a rimmed baking sheet and bake until the surface is golden brown and a cake tester inserted in the center comes out clean, 35 to 45 minutes, depending on the size of the skillet. (The smaller the skillet, the longer the cake will need to be in the oven.) The fruit will sink somewhat during baking and may fall to the bottom and that's okay.

Step 6

Transfer the skillet to a wire rack to cool slightly before slicing into wedges and serving.