You can bake the cupcakes and make the buttercream up to 2 days before decorating. Keep the cupcakes in an airtight container or cupcake keeper. Refrigerate the types of buttercream (chocolate and white) separately in covered containers; when ready to use, whip each one until smooth with an electric mixer and then transfer them to the pastry bags. We recommend you use your favorite commercially prepared eggnog for this recipe; store-bought is more consistent in thickness and is also pasteurized. The buttercream for these cupcakes needs to be thick so it holds its shape well. As you pipe, the warmth of your hands may cause the frosting to become softer. If this happens, put the frosting bags in the refrigerator for 10 minutes to firm up.