Preheat the oven to 200 degrees and line two baking sheets with parchment paper. Beat the egg whites, peppermint extract, and cream of tartar in a stand mixer with the whisk attachment on medium speed until the mixture is frothy and peaks begin to form.

While the mixer still running, add the sugar one tablespoon at a time, waiting a few seconds between each addition.

After all the sugar has been added, add the green gel until you get the desired color. Continue beating for around five more minutes, until the mixture, is glossy and stiff. Transfer the mixture to a piping bag with a large star tip. To make the Christmas tree shape, squeeze three mounds of meringue on top of each other, being careful not to make the mounds too large or the tree will fall over.

After piping all the trees, top each meringue with rainbow sprinkles.

Place the cookies in the preheated oven and bake for 45 minutes. When the cooking time is up, turn off the oven and allow the cookies to sit in the oven for an additional 30 minutes. Then, remove the pan from the oven and allow the cookies to cool completely before eating or storing them in an airtight container. Enjoy!