Preheat oven to 225°F (107°C) and line two baking sheets with parchment paper, preparing them for baking.

In a very clean stand mixer bowl with a whisk attachment, beat egg whites on medium speed until foamy, then add cream of tartar and continue beating until soft peaks form.

Gradually add granulated sugar, one tablespoon at a time, beating on high speed until stiff, glossy peaks form and the sugar is fully dissolved.

Gently fold in vanilla extract and green food coloring until just combined and uniformly colored throughout the meringue.

Transfer the meringue to a piping bag fitted with a star tip and pipe festive tree shapes onto the prepared baking sheets, decorating with sprinkles if desired.

Bake for 90 minutes, then turn off the oven and leave the cookies inside for another hour or until completely cool and dry.

Remove cookies from the oven and cool completely on the baking sheets before storing them in an airtight container at room temperature to maintain freshness.