Place the egg whites in a bowl and allow them to "age" on the countertop for a few hours or over night.
In a mixing bowl, whisk together the confectioners' sugar and almond flour until there are no clumps and the mixture is smooth.
Using a stand mixer or a hand mixer, beat aged egg whites and salt together on low for one minute. Increase the speed to high and beat for 2-3 minutes, or just until the egg whites form stiff peaks.
Gently fold the lemon zest into the whipped egg whites.
Being careful not to deflate the egg whites, gently and slowly fold the granulated sugar into the egg whites one tablespoon at a time.
Once the granulated sugar has been folded into the batter, gently fold half of the confectioners' sugar/almond flour mixture into the batter. Do the same thing with the remaining confectioners' sugar/almond flour mixture. The batter should be the consistency of pancake batter and feel very sticky.
Allow the batter to rest in the bowl for 30 minutes.
Line two baking sheets with parchment paper or silpats. Spoon the batter into a pastry bag with a small round tip. Pipe the batter into circles 1-inch in diameter, ensuring that each shell is an inch apart.
Allow the unbaked shells to rest for another 45 minutes to ensure that the tops of the shells harden.
Preheat the oven to 325 degrees.
Bake the macaron shells one tray at a time for eight minutes, turning the tray halfway through the baking process to ensure even coloring and consistency.
While the shells are cooling, make the filling.
Cream butter, confectioners' sugar, lemon juice and zest for 3-4 minutes until it becomes a fluffy frosting.
Place one teaspoon of filling between every two macaron shells and gently press the shells together to finish the process!