From Elegant but Easy and The Essential New York Times Cookbook,
This is ideal with purple Italian prune plums, but if you can’t find them, other plums will do.
1 cup (125 grams) all-purpose flour
1 teaspoon (5 grams) baking powder
Large pinch of salt
1 cup (200 grams) granulated sugar plus 1 to 2 tablespoon (depending on sweetness of plums)
1/2 cup (115 grams or 8 tablespoons) unsalted butter, softened
2 large eggs
12 smallish purple Italian purple plums, halved and pitted
2 teaspoons fresh lemon juice
1 teaspoon ground cinnamon
Heat oven to 180°C. Sift or whisk together flour, baking powder and salt in a medium bowl. In a larger bowl, cream butter and 1 cup sugar together with an electric mixer until fluffy and light in color. Add the eggs, one at a time and scraping down the bowl, then the dry ingredients, mixing until just combined.
Spoon batter into a greased 9-inch spring form pan and smooth the top. Arrange the plums, skin side up, all over the batter, covering it. Sprinkle the top with lemon juice, then cinnamon, then remaining sugar.
Bake until cake is golden and a toothpick inserted into a center part of the cake comes out free of batter (but of course not plum juice), about 45 to 50 minutes. Cool on rack.
Once cool, if you can stand it, and I highly recommend trying, leave it covered at room temperature overnight as this cake is even better on the second day, when those plum juices further release into the the cake around it, becoming not just “cake with plum,” but cakeplumughyum (official terminology, there).