PREHEAT oven to 350°F. Place Reynolds® Foil Baking Cups in muffin pans or on a cookie sheet; set aside.
PREPARE cake mix following package directions for 24 cupcakes, adding 2 tablespoons bourbon to batter. Spoon batter into baking cups.
BAKE 18 to 22 minutes. Cool.
MIX remaining bourbon, butter, milk and vanilla extract until blended. Gradually add powdered sugar until blended. Stir in walnuts. Frost cupcakes.