Preheat oven to 200 degrees F.
In your stand mixer bowl fitted with the whisk attachment combine egg whites, cream of tartar, and salt. Whip on medium speed until foamy and light in color.
Add vanilla extract and 1-2 drops of green food coloring. Slowly add sugar 1 tablespoon at a time. Allow 10-15 seconds of whipping between additions. Continue until all sugar is added and whip until the meringue is glossy and stiff. When you remove your whisk attachment the meringue batter should stand up and just barely tip over to one side. Sugar should be dissolved at this point. Test by rubbing a bit in-between your fingertips.
Transfer the meringue to a clean piping bag fitted with a star tip. Pipe trees onto a parchment-lined baking sheet. Top with sprinkles if using.
Bake meringue cookies at 200 degrees F for 1 hour. DO NOT OPEN THE OVEN DOOR AT ANY POINT.
With the oven door still closed turn off the oven. Let the meringue cookies cool completely and continue to dry out for at least 1 more hour up to overnight.
Store in an air tight container in a cool dry place 1-2 days.