Place the candy melts in separate bowls and heat, according to package instructions for the microwave or double boiler. Be careful not to overheat or get water into the bowls.
Gently insert a toothpick or fork between the two cookie layers. Dip the cookie into the melted candy, coating all sides. Tap off the excess coating and place cookie on parchment-lined baking sheet. Repeat to make 12 red and 12 white dipped cookies.
Let the cookies sit until the coating is fully hardened. I like to pop them into the refrigerator for 10-20 minutes to help the coating set.
Use a small round piping tip with the tube of green icing to draw a squiggle in the center of the cookie to make a “tree.” Start with a small back and forth motion for the top of the tree, moving in larger swirls down to the tree's base.
Sprinkle nonpareils over the freshly piped icing and place a star candy at the top.
Store cookies in a single layer in airtight containers at room temperature for 1-2 weeks.
Candy melts and decorator's icing tubes are available in the baking aisle of many grocery stores, and at craft stores. *If making these cookies with or for children, check the recommended consumption age on the packages of any decorative sprinkles/candies you buy, in conjunction with your best judgment, to ensure that they don't present a choking hazard for little ones. If you find that the candy melts start to thicken as you're dipping the full batch of cookies, reheat them in the microwave at half power for 15- second increments, stirring.