Place egg whites in a small bowl (make sure there is no trace of egg yolk). Cover with plastic wrap and poke a few large holes in it. Leave out at room temperature for 8 hours or in the refrigerator overnight. Bring to room temperature before using.
Place confectioners' sugar and fine almond flour into a fine mesh sieve over a bowl. Sift and let the bowl catch the sifted ingredients. Discard any large clumps in the sieve. Repeat twice and set aside dry ingredients.
Put egg whites in a very clean bowl of a stand mixer fitted with a whisk attachment. Whisk eggs on medium-low speed for a minute or two until frothy then add the cream of tartar and salt. With the mixer running, very slowly add the bakers sugar (either 2 teaspoons at a time or in a very slowly sprinkle into the bowl). Increase speed to medium-high and beat until stiff peaks form, about 5 minutes on KitchenAid Stand Mixer speed 6. Pro tip: The mixture will ball up in the whisk and the mixture will stay in the bowl if turned upside down.
Add half of the dry ingredients to the mixing bowl. Use a silicone spatula to gently fold the dry ingredients into the wet ingredients. Once mostly incorporated, add the remaining dry ingredients. Continue folding by going around the sides of the bowl with the spatula, then cutting through the center. The batter is done when it has a lava-like consistency. Do not over-mix. Pro tip: Try lifting the spatula out of the bowl and forming figure 8 with the batter without it breaking.
Transfer batter to a piping bag fitted with a round tip (about 1/2-inch in diameter). Pipe batter onto a silpat mat with macaron outlines or onto parchment paper. Pipe circles 1-inch wide and spaced 2 inches apart. Lift the baking sheet about 6 inches off the counter and drop it down. Repeat twice more, then rotate the pan 180° and bang the pans three more times (this releases air bubbles from the macarons).
Let the macarons sit at room temperature for 40 to 45 minutes and up to an hour. The macarons are ready to bake when dry to the touch (you should be able to "pet" them without leaving a mark).
Ten minutes before macarons are done drying, preheat the oven to 300° F. Bake macarons for 12 to 14 minutes, rotating the pan halfway through. Cool completely.
Match up the cooled macarons according to size. Make your favorite filling, such as buttercream frosting, and pipe the filling between the cookies. Serve and enjoy!
Notes
Bakers sugar is a superfine granulated sugar. If you can't find bakers sugar, use granulated sugar and process it in a food processor for ten 1-second pulses.Store macarons in the refrigerator for 5 days. If using a liquid-y filling such as curd or jam, assemble macarons the day you expect to serve.Freeze macarons (assembled or unassembled) in an airtight container for up to 3 months. Place parchment paper between layers so cookies do not stick.