Place egg whites in a medium bowl, and let stand at room temperature for exactly 3 hours. (Aging the egg whites in this manner is essential to creating perfect macarons.)
Line 2 to 3 rimmed baking sheets with nonstick baking mats. Using a permanent marker, draw 1½-inch circles 1 inch apart on a sheet of parchment paper. Place template under 1 nonstick baking mat.
In the work bowl of a food processor, place confectioners’ sugar, almond flour, and salt; process until well combined and mixture is a uniform, fine texture, scraping down sides of bowl as needed. Sift confectioners’ sugar mixture twice using a fine-mesh sieve; use a spatula or a whisk to press mixture through sieve as needed. (If you do not have a food processor, sift together confectioners’ sugar, almond flour, and salt three times.)
In the bowl of a stand mixer fitted with the whisk attachment, beat egg whites at medium-high speed until foamy, 30 seconds to 1 minute. Add granulated sugar in a slow, steady stream, scraping down sides of bowl as needed. Increase mixer speed to high, and beat until stiff peaks form, about 1 minute. Gently fold in confectioners’ sugar mixture in thirds, adding extract and food coloring with last addition. (Properly folded macaron batter will move in a fluid, slow-moving ribbon and will be able to make a figure 8.)
Transfer batter to a pastry bag fitted with a ½-inch round tip (Wilton No. 2A). Holding tip perpendicular to pan, pipe batter onto drawn circles of template underneath mat. Apply pressure to bag, leaving tip stationary, until batter reaches drawn circle. Release pressure and move piping tip in a quick circular motion as you finish piping each macaron shell to prevent a point from forming on top. Remove template carefully and place under second baking mat to pipe batter. Repeat procedure with remaining batter. Tap pans vigorously on counter several times, rotating pan 180 degrees each time, to release air bubbles; use a thin wooden pick to pop and fill any large bubbles, tapping pans firmly again to smooth. Let stand at room temperature until a skin forms on top of macaron shells, 45 minutes to 1 hour. (Batter should feel dry to the touch and should not stick to your finger.)
Preheat oven to 275°.
Bake, one pan at a time, until shells are firm to the touch, about 15 minutes, rotating pan every 5 minutes. Let cool completely on pans.
Place White Chocolate-Mint Frosting in a pastry bag fitted with a ½-inch round tip*. Pipe buttercream onto flat side of half of macarons. Place remaining macarons, flat side down, on top of buttercream, and gently press together. Macarons are best enjoyed the same day they are made but can be refrigerated in an airtight container for up to 5 days.
Notes
*We used Nielsen-Massey Pure Peppermint Extract, Wilton Leaf Green Food Coloring Icing Color, and a Wilton No. 2A decorating tip.
Recipe by Victoria at https://victoriamag.com/white-chocolate-mint-macaron-recipe/