Preheat the oven to 450 degrees F (230 degrees C). Grease a 9-inch square baking dish.

Beat sugar, butter, and eggs with an electric mixer until smooth and creamy. Mix in pumpkin purée until well combined. Add milk, cake flour, and baking powder and mix until smooth. Pour mixture into the prepared baking dish and sprinkle cinnamon over top.

Bake in the preheated oven until set in the middle, 45 minutes to 1 hour.