Preheat oven to 350°F. Mix 1/2 cup of the flour, cinnamon and baking powder in medium bowl. Set aside. Toss chopped candy bars and remaining 2 tablespoons flour in small bowl. Set aside.
Mix butter and brown sugar in large bowl until well blended. Add egg yolk and vanilla; mix well. Add flour mixture; mix just until moistened. Shake chopped candy bars to remove excess flour. Stir into batter. Divide among four ovenproof 8-ounce coffee mugs. Place mugs on baking sheet.
Bake 25 to 30 minutes or until toothpick inserted into center comes out almost clean. Let stand 10 minutes before serving. (Blondie may fall in center upon cooling.) Serve with vanilla ice cream or whipped cream, if desired.