Preheat the oven to 350°F Line a 12-cup muffin tin or two 6-cup muffin tins with paper liners.
In a medium bowl, whisk the flour, spice, baking powder, baking soda, and salt. In a large bowl, whisk the pumpkin, brown sugar, eggs, butter, and sour cream. Fold in the flour mixture, walnuts, and cranberries until just combined. Divide among muffin cups.
Bake until a toothpick inserted in the center of a muffin comes out clean, about 25 minutes. Cool in the pan on a wire rack for 5 minutes, then cool on the rack.