Preheat the oven to 350 degrees F (175 degrees C). Grease and flour a 7x11-inch baking dish.

Beat 1/2 cup sugar and 2 tablespoons butter together in a bowl with an electric mixer until smooth and creamy. Add 1 cup flour, eggs, baking powder, and salt. Beat in vanilla. Pour batter into the prepared baking dish. Add plums, skin side down, over top.

Mix 1 cup sugar, 1/2 cup flour, 3 tablespoons melted butter, and cinnamon together in a separate bowl until crumbly: spoon over batter and plums.

Bake in the preheated oven until plums are softened, about 35 minutes.