Preheat oven to 225°F (110°C) and line baking sheet with parchment paper.
In clean bowl, whisk egg whites on medium speed until foamy, about 2 minutes.
Add cream of tartar and continue whisking until soft peaks form.
Increase to high speed and gradually add sugar one tablespoon at a time until stiff, glossy peaks form, 8-10 minutes total.
Gently fold in vanilla and green food coloring with spatula until just combined.
Transfer to piping bag with large star tip. Pipe tree shapes by starting with wide base and spiraling upward in decreasing circles to create point at top.
Bake 1 hour 30 minutes without opening oven door until completely dry and crisp.
Turn off oven, leave door slightly open, and cool completely inside for 30 minutes before removing.
Notes
Room temperature egg whites whip to better volume. Wipe bowl with vinegar to ensure it’s grease-free.
Gel food coloring works better than liquid – won’t thin the meringue.
Practice piping technique on parchment before final batch. Hold bag at 90-degree angle and spiral from outside to center.
Store in airtight container at room temperature up to 5 days. Never refrigerate as humidity makes them chewy.