My husband likes plums and any kind of cake so I decided to try this. It was easy and could be eaten for breakfast like he did because it is not too sweet. I used a 13" skillet because that is the only size skillet I have and did a recipe and a half but the recipe below is for a 8" or 9" skillet. I found this recipe at marthastewart.com.
4 tbsp unsalted butter, at room temperature, plus more for skillet
1 cup all purpose flour, plus more more skillet
1/2 tsp baking powder
1/4 tsp baking soda
3/4 cup plus 2 tbsp sugar
1 large egg
1/2 cup low fat buttermilk
2 ripe medium plums, thinly sliced
Preheat oven to 375 degrees. Butter an 8" or 9" skillet preferably cast iron and dust with flour, tapping out excess. You can also use a 8" or 9" cake pan if you don't have a skillet. Whisk together flour, baking, baking soda and 1/4 tsp salt.
Beat butter and 3/4 cup sugar with a mixer on medium until pale and fluffy. Beat in egg. Add flour mixture in 3 additions, alternating with buttermilk.
Pour batter into prepared skillet and smooth top. Fan plums out and sprinkle with remaining 2 tbsps of sugar. Bake until golden brown and a toothpick inserted into the center comes out clean - about 30-35 minutes. Let cool slightly.