Preheat the oven to 200°F/93°C. Line two baking sheets with parchment paper and set aside.
To the bowl of a stand mixer, add the room temperature egg whites and the cream of tartar. Beat with the whisk attachment on low speed until the mixture becomes foamy. Increase the speed to high, and add the sugar, 1 tablespoon at a time, mixing for about 15 seconds between each addition. Once all the sugar is added, continue beating until the egg whites reach the stiff peaks stage and are thick and shiny.
To check to see that the meringue is ready, you can rub a small amount of it between your fingers to make sure that it’s not gritty. If there is any grit, continue mixing until it’s gone. You should also lift the whisk up out of the bowl to see that the peaks are indeed stiff, with peaks that will curl over on themselves.
Add food coloring and vanilla extract and mix until the color is evenly blended.
Transfer the meringue to a large piping bag fitted with a medium or large piping tip. You can use a round, flat, or even an open star tip. I suggest a Wilton 18 Star or a Wilton 12 round for these, but you can use any tip you like as long as you can draw lines with it.
Pipe the meringue onto the parchment lined sheets, making lines in a back and forth pattern to create a triangular tree. Each tree should be no larger than 3 inches tall. You can put marks on the underside of the parchment to guide if you like.
Add the sprinkles before baking. I like the stars at the top and then colorful sprinkles to act as decorations.
Bake the meringues for 1 hour at 200°F/93°C, then turn off the oven. DO NOT OPEN THE DOOR at any point.
Leave the meringues in the oven and allow the oven to cool down completely before removing the cookies. This can take between 1-3 hours. The cookies should be crisp and dry when finished.