Preheat oven to 450 degrees F (230 degrees C). Grease a 9-inch square baking dish.
Beat sugar, butter, and eggs together in a bowl using an electric mixer until smooth and creamy; add pumpkin and mix well. Beat milk, flour, and baking powder into pumpkin mixture until smooth; pour into the prepared baking dish and sprinkle cinnamon over the top.
Bake in the preheated oven until set in the middle, 45 minutes to 1 hour.
