Preheat oven to 400°F. Spray muffin pan with cooking spray.
Combine flour, sugar, baking powder, cinnamon and salt in a large bowl; set aside. Whisk egg, pumpkin, orange juice, oil and vanilla in a medium bowl until well mixed. Add to flour mixture and stir until flour is moistened. Fold in walnuts.
Evenly divide batter into prepared muffin cups, filling each about two thirds full. If desired, garnish with chopped walnuts and coarse sugar, pressing gently into top of batter.
Bake 15 to 20 minutes for regular size muffins or 20 to 25 minutes for large muffins, or until puffed and deep golden brown. Transfer muffins to a rack to cool slightly. Serve warm.