Preheat your oven to a gentle 200°F (95°C). In a clean, heat-proof bowl of your stand mixer, combine the egg whites, granulated sugar, and cream of tartar. Set this bowl over a pot of simmering water (make sure the water doesn't touch the bottom of the bowl). Whisk continuously and vigorously until all the sugar crystals have completely dissolved and the mixture feels smooth to the touch. This step is key for a stable meringue! Remove the bowl from the heat and stir in the almond extract.
Using the whisk attachment on your stand mixer, beat the mixture on medium-high speed. Continue mixing until the egg whites become brilliantly white, glossy, and form firm, stiff peaks that hold their shape beautifully. Now, carefully fold in your green gel food coloring until you achieve your desired festive shade of green.
Prepare a baking sheet by lining it with parchment paper or a silicone mat. Transfer your fluffy green meringue mixture into a piping bag fitted with a large star tip. Gently pipe small Christmas tree shapes onto your lined baking sheet, aiming for about 1 inch wide and 2 inches high, leaving about an inch of space between each tree. Sprinkle generously with the multi-color sprinkles for a festive touch!
Place your baking sheet into the preheated oven and bake for 2 hours. After the baking time, turn off the oven completely. Prop the oven door open slightly (you can use a wooden spoon) and allow the cookies to cool and dry out in the oven for an additional 2 hours. This gentle, slow cooling prevents cracking and ensures they are perfectly crisp and melt-in-your-mouth light!
Once your meringue trees are completely cool, carefully remove them from the baking sheet. To finish, add a tiny dab of frosting to the very top of each "tree" and gently place a star sprinkle on it. Serve these delightful, airy, and fat-free treats and spread some holiday cheer!