1. Combine plums, 1 tablespoon sugar, lemon juice, and 1/4 teaspoon kosher salt in a bowl, tossing gently.
2. Grease sides and bottom of a 9-inch cast-iron skillet or deep-dish pie pan. Cover bottom with plum slices, overlapping slightly.
3. Whisk together eggs, remaining sugar, buttermilk, vanilla, and butter until well blended.
4. Combine flour, baking powder, and remaining 1/2 teaspoon kosher salt; whisk into egg mixture. Pour batter over fruit. Bake at 375° for 20 to 25 minutes or until golden brown. Cool in pan about 30 minutes.
5. Run tip of knife around edge of cake to loosen. Invert onto a platter, and serve with sweetened whipped cream.