Heat the oven to 375 degrees F and line a standard 12-cup muffin tin with paper liners. Combine the spice cake mix and the pumpkin puree in a large bowl. Divide the batter between the lined muffin cups (mixture will be thick). Bake until a toothpick inserted near the center of a muffin comes out clean, 18 to 20 minutes. Cool slightly on a wire rack. Serve warm with lots of butter.