Place a nonstick silicone baking mat or parchment paper on a baking sheet. (See notes below.) This recipe will make enough macarons for several baking sheets.
Using a food processor, combine the almond flour and confectioners' sugar and process until the mixture is fine. Then, sift the flour mixture, discarding any large almond pieces if necessary. Set aside.
Using a mixing bowl and the whisk attachment, beat 6 egg whites until foamy. Add 1/3 of the sugar and beat for 1 minute until the sugar dissolves. Add half of the remaining sugar and beat for 1 more minute. Then, add the rest of the sugar and beat until firm, glossy peaks form, about a minute or two.
Add the green apple flavoring. For green pastel-colored cookies, add a small amount of green gel coloring and a little yellow, and beat until well mixed.
Use a spatula to gently fold the dry ingredients into the egg whites. Mix well. The batter will be thick. Then, with a hand whisk, beat the 1/2 egg white until frothy. Stir this egg into the almond mixture to soften and moisten the batter.
Snip off the end of the large piping bag and place the round tip into it. Then, place the batter into the bag. Fold the top of the piping bag over, then roll it down and hold the top so no batter can escape. Place the bag over the baking sheet and position it vertically upright. Pipe 1 to 1 1/2-inch round circles onto the mats or parchment paper. Leave room between each circle for the batter to spread.
Tap each baking sheet on the counter as you finish piping a full pan. Tapping will spread the macaron batter and also help remove air bubbles. Use a toothpick to burst any bubbles in the macarons. Let the macarons sit for about 30 minutes to form a dried skin on top. In dry climates, they might dry out in 10 minutes. In very humid climates, it might take longer. Test the macaron skins with your fingertip. If the batter sticks to your finger, they are not ready and should sit longer. If no batter sticks to your finger, they are cured for baking.
Preheat the oven to 300°F (150°C). Bake the macaron shells for 14-15 minutes. Do not brown them or they will lose their color. Cool them completely after baking.
Gently remove the macarons by pulling the mat (or parchment paper) away from the shells. Place the shells flat side up (upside down) on a large cutting board or another flat surface. This will make it easier to add the filling to them, and they will also be easier to pick up.