Preheat oven to 180 degrees C. Grease a 20cm square baking dish.
Spread the plums out into the prepared baking dish and sprinkle with 1/4 cup sugar. In a bowl, mix together remaining 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.