Preheat oven to 350 degrees Fahrenheit. Lightly coat thirty-two 1 3/4-inch muffin cups with cooking spray or line with paper bake cups. In a medium bowl, stir together flour, sugar, cocoa powder, baking soda, and salt. In another bowl, whisk together pumpkin, egg, oil, and vanilla. Add to flour mixture; stir just until combined. Spoon into prepared muffin cups, filling two-thirds to three-fourths full. If desired, sprinkle with nuts and/or chocolate chips. Bake 12 to 15 minutes or until a toothpick inserted in center comes out clean. Cool in a pan on a wire rack at least 20 minutes. Serve warm or remove from pan and cool completely.