In a large bowl, cream butter and brown sugar until light and fluffy. Add eggs, one at a time , beating well after each addition. Combine the flour , baking powder, baking soda, cinnamon and allspice; gradually add to creamed mixture alternataly with root beer, beating well after each addition.
Fill paper-lined muffin cups two-thrids full. Bake at 350* for 18-22 minutes or until a toothpick comes out clean. cool for 10 minutes before removing from pan to wire rack to cool completely.
Just before serving, combine whipped topping and crushed candies; frost cupcakes. Refrigerate leftovers. Yields 1 dozen