Preheat oven to 350°F. Coat a 15x10x1-inch baking pan with cooking spray. Gently wipe the pan interior with a paper towel so the pan bottom and sides are evenly coated with the spray. Dust with the 2 teaspoons all-purpose flour; set aside.
In a large bowl sift together the 1½ cups all-purpose flour, the granulated sugar, whole wheat flour, the 3 tablespoons cocoa powder, the baking soda, salt, and pumpkin pie spice.
In a medium bowl combine pumpkin, yogurt, egg, canola oil, food coloring, the 1½ teaspoons vanilla, and the vinegar. Add to flour mixture, stirring just until combined. Spread into the prepared baking pan.
Bake for 25 to 30 minutes or until a toothpick inserted near the center comes out clean. Cool in pan on a wire rack. Cut into 24 bars.
For topping, in a medium bowl combine topping mix, cream cheese, milk, powdered sugar, and the ½ teaspoon vanilla. Beat with an electric mixer on high speed until stiff peaks form (tips stand straight). Cover and chill until needed.
To serve, top each bar with a tablespoon-size dollop of the topping. If desired, sprinkle with additional cocoa powder.
Tips: Sugar Substitutes: Choose from Splenda Sugar Blend for Baking or C&H Light Sugar & Stevia Blend. Follow package directions to use product amount equivalent to 1⅓ cups granulated sugar. PER SERVING WITH SUBSTITUTE: same as below, except 120 calories, 18 g carbohydrate, 8 g sugar Exchanges: 1 starch Carb Choice: 1