Special equipment: a 2-inch Christmas tree cookie cutter and a piping bag with a 1/16-inch tip, optional
For the cookies: Whisk together the flour, baking soda and salt in a medium bowl. Mix the mint and vanilla extracts with the egg in a small bowl with a fork.
Beat together the butter, granulated sugar and confectioners' sugar in the bowl of a stand mixer on low speed until the butter has picked up the sugars, about 30 seconds. (If using an electric hand mixer, beat about 2 minutes.) Increase the speed to medium and beat, stopping halfway to scrape the bowl, until slightly creamy, about 1 minute (about 3 minutes with a hand mixer).
Reduce the speed to low, slowly add the egg mixture and beat until combined. Add the flour mixture a heaping cup at a time in 2 additions, stopping a few times to scrape the bowl and beaters. (If using a hand mixer, increase the speed as the dough gets thicker to keep the beaters spinning.) Once all the flour is just incorporated, increase the speed to medium (higher with a hand mixer) and beat until the dough is very smooth, about 2 minutes (about 5 minutes with a hand mixer).
Turn the dough out of the bowl and bring it together. Divide the dough into 2 even pieces, each about 11 ounces. Shape each piece into a 6-inch square and wrap in plastic wrap. Refrigerate the dough for at least 3 hours and up to overnight.
Remove one of the dough squares from the refrigerator and let soften until just pliable but still cool and firm, about 15 minutes. Position oven racks in the top and bottom thirds of the oven and preheat to 350 degrees F. Line 2 baking sheets with parchment.
Dust a piece of parchment with flour. Dust the dough square with flour and roll it out on the parchment to a thickness of about 1/4 inch. Lift the dough occasionally by sliding a metal spatula or bench scraper underneath to make sure it isn't sticking to the parchment. Cut out Christmas trees with a 2-inch cutter as close together as possible and transfer them to the prepared baking sheets, leaving 1 inch between the trees. Gather together the scraps and roll and cut more cookies. Chill the cookies on the baking sheets in the freezer for 10 minutes before baking.
Bake, rotating the baking sheets from front to back and top to bottom halfway through, until the cookies are light golden brown, 18 to 20 minutes. Cool the cookies about 5 minutes on the baking sheets, then transfer them to a wire rack to cool completely, about 30 minutes.
Meanwhile, remove the second dough square from the refrigerator to soften until just pliable but still cool and firm, about 15 minutes. Repeat the rolling, cutting, chilling and baking procedures with the second dough square.
For the green royal icing: Combine the confectioners' sugar and meringue powder in a large bowl. Add 3 tablespoons water and beat with an electric mixer on low speed until the frosting thickens. The icing should be pure white and thick but not fluffy and bubbly. (If the frosting is overbeaten, it will get aerated, which makes it harder to work with. If this happens, let the frosting sit for a bit to settle, then use a rubber spatula to vigorously beat and smooth out the icing.) Add the green and red food coloring and beat until evenly distributed and the color is a deep hunter green. Put the icing in a resealable plastic bag and snip the corner to make a tiny opening (or put the icing in a piping bag with a 1/16-inch tip).
To decorate: Pipe a dot of icing onto the top point of each tree. Stick a chocolate nonpareil candy on each dot. Pipe the icing onto the cookies in zigzags to make a tree design. Sprinkle white nonpareils around the icing as snow. Let the icing set at room temperature, at least 1 hour.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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