Preheat oven to 350°F. Grease a 10-inch round cake pan or springform pan with cooking spray, set aside.

Cream together the butter and sugar until light and fluffy, about 3 minutes. Beat in the vanilla and eggs. Stir in the flour, baking powder, and salt until just combined. Transfer the batter to the prepared pan and spread out into an even layer. Place the plum halves skin side up on top of the batter pretty close to on another. Sprinkle with the sugar and the cinnamon.

Bake until golden brown and the center has just set, about 45 minutes. Remove from the oven and allow to cool in the pan. Slice and serve with a dollop of whipped cream, powdered sugar or ice cream. Leftovers can be stored in the fridge for up to 3 days. Enjoy!