Preheat oven to 350F. Line 24 (2 1/2") muffin cups with cupcake liners. In a large bowl, beat together cake mix, root beer, oil and eggs on low speed for 30sec. Scrape down side of bowl; beat 2mins on medium. Spoon batter into liners, filling each about 2/3 full.
Bake 18-22mins or until toothpick inserted into center comes out clean. Cool cupcakes in muffin cups on a wire rack 5mins. Transfer cupcakes from pans to racks; cool completely.
In medium bowl, stir together frosting and root beer extract until well mixed. Spread each cupcake with root beer frosting. If desired, dip edges of each cupcake into crushed candies. If desired, remove and discard bottom halves of straws. Insert 1 straw top into each cupcake.