Preheat and prep: Heat your oven to 350°F (175°C). Line a 12-cup muffin tin with liners or lightly grease it.
Mix the batter: In a large bowl, combine the spice cake mix and pumpkin puree. Stir with a spatula until just blended.
The batter will be thick—this is normal.
Fill the muffin cups: Scoop the batter evenly into the muffin tin, filling each cup about 3/4 full. A cookie scoop makes this easy.
Bake: Bake for 16–20 minutes, or until the tops spring back when lightly pressed and a toothpick comes out clean or with a few moist crumbs.
Cool slightly: Let the muffins sit in the pan for 5 minutes, then transfer to a wire rack.
Make the coating (optional but recommended): Mix sugar and cinnamon in a shallow bowl. If using butter, brush the warm muffin tops lightly with melted butter.
Coat: Roll the warm muffin tops in cinnamon sugar until evenly coated.
For full coverage, roll the entire muffin.
Serve: Enjoy warm or at room temperature. The texture is soft and cakey, with a cozy spice aroma.