Preheat the oven to 350 degrees F. Mist the cups of a standard nonstick muffin pan with I Can't Believe It's Not Butter! spray. Set aside.
In the bowl of a food processor fitted with the metal blade, process the flour for about 1 minute until no coarse grains remain.
In a mixing bowl, combine the flour, pumpkin pie spice, cinnamon, baking soda, baking powder, and salt. Sift the mixture twice. Set aside.
In a large mixing bowl, combine the yogurt and egg whites. With a sturdy whisk, beat until thoroughly blended. Add the sugar substitute and the pumpkin. Whisk to blend.
Add the reserved flour mixture, stirring, until no four is visible.
Spoon the batter into prepared cups until each cup is about two-thirds full. Sprinkle on walnuts.
Bake for 25 to 30 minutes, or until a toothpick inserted into the center of one muffin comes out dry.
Cool in the pan on a rack for 10 minutes. Transfer the muffins to the rack. Serve warm or cooled.
Source: Devin Alexander from the Biggest Loser Cookbook