Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Place plums into the prepared baking dish.
Beat 1 ½ cups white sugar and ⅔ cup butter together in a large bowl with an electric mixer until smooth and creamy; beat in eggs, one at a time, beating well after each addition. Stir in 3 cups sifted flour, baking powder, and salt; stir in milk and vanilla extract until smooth. Pour batter over plums.
Combine remaining 1 cup sugar, ½ cup butter, ¼ cup flour, and cinnamon in a bowl until it resembles coarse crumbs; sprinkle over batter.
Bake in the preheated oven until a toothpick inserted into center comes out clean, 45 to 50 minutes.
