In a large bowl, whisk together sugar, pumpkin, egg, vanilla and melted butter. Sprinkle cinnamon, nutmeg and baking powder, and stir in. Add in flour, mix until thoroughly combined.
Pour into greased pan. Place in preheated oven; bake for 20-23 minutes, or until a toothpick tester is removed cleanly.
Spread the top of the warm cake with butter. Sprinkle liberally with cinnamon sugar.