Find a small saucepan that your stand mixer bowl can sit on top of to create a double boiler (bain marie). Add water to the saucepan and bring to a boil.
Add egg whites, granulated sugar, and cream of tartar to a stand mixer bowl. Whisk to roughly combine. Once the water is boiling, reduce the heat to medium-high. Place the stand mixer bowl on top of the water to create a double boiler (bain marie). Whisk the egg mixture, stirring constantly, until the mixture reaches 100°F (38°C) on an instant-read thermometer, about 30 seconds - 1 minute. Carefully remove the hot bowl from the heat, and transfer it over to the stand mixer.
Whip on medium-high speed until stiff peaks form. Optionally, add 5 - 10 drops of gel food coloring once the egg whites reach soft peaks stage. Once you reach stiff peaks, you'll notice the meringue starts to ball up inside the whisk and is glossy. To test the meringue stiffness, pull the whisk out. A stiff peak should stand straight up (no curl at the tip). You'll also know it's ready because you can fully flip the bowl over, and nothing moves or falls out. Make sure not to overbeat the meringue at this stage (this is when the egg whites look dull or curdled).
Pour the sifted almond flour & confectioners' sugar into the meringue mixture, and gently fold them together using a silicone spatula. Continue until all the meringue is fully incorporated, and no dry bits of almond flour remain on the bottom of the bowl.
Begin working the batter until everything is fully incorporated (this is called the macaronage process). Slowly spread the mixture along the side of the bowl, then wipe it back down. Be sure to do this gently. Repeat until the mixture has reached a smooth, shiny, flowing consistency. You can test this by lifting your spatula and seeing if the batter drops almost like lava. Once the batter drops back into the bowl, it should take about 10 - 15 seconds for the ribbon that formed to "disappear" back into the rest of the batter. Don't over macaronage the batter or the macarons will not properly bake in the oven.
Pour the batter into the prepared piping bag. Hold the bag vertically, and gently pipe 1.5 inch (3.8 cm) circles. Once you have reached the desired size, release the pressure on the bag and twist your wrist without lifting up. Continue until no batter remains.
Tap the sheet pan down the counter a few times to release any air bubbles. Pop the air bubbles with a toothpick. Allow the batter to sit on the counter and dry for 60+ minutes until a thin "skin" develops, and you can touch the tops without ruining the macaron. This resting time will vary depending on the humidity (they may take up to 90+ minutes to dry if you live in a humid or rainy environment). While the macarons are drying, heat oven to 300°F (150°C).
Bake one sheet at a time on the center rack of the oven for 19 - 23 minutes. You can test that the macarons are done by gently touching the edge and the feet of the macarons. If the shell wiggles, is sticky, or wet, the macarons need to bake longer. Remove macarons from the oven, and move the sheet pan to a cooling rack until completely cooled.
Match up macaron shells so that they are paired with a similar size/shaped shell. Pipe your desired filling on one side of the macaron shell. Gently place the matching side on top of the filling.
Mature the macarons with the filling in the refrigerator overnight. This will allow the macarons to slightly soften and develop a deeper flavor.