1-Preheat the oven to 200 degrees Fahrenheit: Preheat the oven to 200 degrees Fahrenheit and line two baking sheets with parchment paper.
2-Add the egg whites and cream of: Add the egg whites and cream of tartar to the bowl of a stand mixer. Beat with the whisk attachment on medium speed until frothy. Ensure your bowl and whisk attachment are clean, dry, and free of any residue before beginning.
3-Gradually add the sugar, 1 tablespoon at: Gradually add the sugar, 1 tablespoon at a time, while continuing to beat. Once all the sugar is added, increase to high speed and beat until stiff, glossy peaks form and the sugar is fully dissolved.
4-Beat in the vanilla extract just until: Beat in the vanilla extract just until combined.
5-Add green gel food coloring and mix: Add green gel food coloring and mix until evenly tinted to your desired shade.
6-Transfer the meringue to a piping bag: Transfer the meringue to a piping bag fitted with a large star tip. Pipe tall swirls in a tree shape onto the prepared baking sheets, spacing them about 1 inch apart.
7-If using, add round sprinkles as ornaments: If using, add round sprinkles as ornaments and top each tree with a star sprinkle.
8-Bake for 45 minutes, then turn off: Bake for 45 minutes, then turn off the oven and let the meringues cool inside the oven for at least 2 hours or preferably overnight. Do not open the oven door during this time.
9-Once fully cooled, store in an airtight: Once fully cooled, store in an airtight container at room temperature.