Mix butter, cinnamon, and salt together in a bowl.
Place pumpkin seeds in a large bowl and pour butter mixture over seeds; toss to evenly coat. Spread coated seeds in a single layer onto a baking sheet.
Bake in the preheated oven, stirring occasionally, until seeds are lightly browned, about 40 minutes. Remove baking sheet from oven; sprinkle sugar over seeds and stir until evenly coated.
