Line a baking sheet with a nonstick silicone baking mat or parchment paper. (See notes below to determine the best option.) This recipe will make enough macarons for several baking sheets.
Using a food processor, combine almond flour and confectioners' sugar. Process until the mixture is finely ground. Sift the blended mixture, discarding any large almond pieces. Set aside.
Using a mixing bowl and the whisk attachment, beat 6 egg whites until foamy. Add the cream of tartar and beat to dissolve it.
Add 1/3 of the granulated sugar and beat for a minute until the sugar dissolves. Add half of the remaining sugar and beat for an additional minute. Then add the rest of the sugar and beat until firm, glossy peaks form, a couple of minutes or more.
Mix in the lemon zest, essential oil, and a small amount of yellow gel coloring until well combined.
Use a spatula to gently fold the dry ingredients into the egg whites. Mix well. The batter will be thick.
In a separate bowl, use a hand whisk to beat the 1/2 egg white until it is frothy. Stir this egg into the almond mixture to moisten the batter.
Snip the end of an extra-large piping bag and insert a round tip. Fill the bag with the macaron batter, fold the top to prevent spillage, and position it vertically over the baking sheet. Pipe 1 1/2-inch circles, leaving space between each for spreading.
Gently tap each baking sheet on the counter as you finish piping a whole pan to spread the batter and remove air bubbles. Use a toothpick to burst any remaining bubbles. Allow the macarons to sit for about 30 minutes to form a skin on top. In dry climates, they may dry out faster, while in humid climates, it could take longer. Test for readiness by touching the surface. 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 when the macarons are near the end of their curing time. Bake them for 14-17 minutes. Do not brown them, or they will lose their color. Cool them completely after baking.
Remove the macarons gently by pulling the mat (or parchment paper) away from the shells. (They should not be sticky underneath, or they are underbaked.) Place the shells flat side up (upside down) on a large cutting board or another flat surface, matching the same-sized shells together.