Preheat oven to 350 degrees F (175 degrees C). Grease an 8x8-inch baking dish.
Spread the plums out into the prepared baking dish and sprinkle with 1/4 cup sugar. In a bowl, mix together 3/4 cup white sugar, flour, baking powder, and salt. Combine with beaten egg and spoon the mixture over the plums. Drizzle batter with melted butter.
Bake in the preheated oven until the topping is brown, about 40 minutes.