Preheat oven to 300ºF and line a 1/2 baking sheet with the macaron template and parchment paper or use a silicone macaron mat with built in template
Sift together the powdered sugar, salt, and almond flour, twice if not blended.
Pulse the mixture in a food processor 8-10 times to make the almond flour mixture even finer and texture and to blend the ingredients together.
Whip the egg whites on low to frothy consistency and slowly add the sugar in thirds while mixing on low.
Once the egg whites turn white and you can see some lines forming in the surface from the whisk, add cream of tartar, whipping on medium until soft glossy peaks form.
Add the vanilla (and food coloring if desired) to the meringue during the soft peak stage. Then continue whipping on medium-high until you get stiff but shiny peaks that start gathering and bunching on the inside of the whisk.
Add 1/3 of your almond mixture to the meringue. Fold your spatula under the batter and around the edges and then cut through the center until almond flour is mixed in. Continue with the rest of the almond flour and folding until homogenous. (see video)
Gently press the spatula on top of the batter while you turn the bowl to take out some fo the air from the meringue. Continue folding around the outside edge until the batter forms a ribbon and moves like lava.
Your meringue is ready when it forms a ribbon off the spatula and the batter that settles almost dissolves all the way back into the rest of the batter but still leave a bit of a line.
Place parchment paper onto your sheet pan. Pipe small rounds about 1" in diameter.
Drop the pan onto the table 5-6 times from about 5" above the table to release bubbles. Use a toothpick to remove big pockets of air trapped under the surface. Use a very small amount of water on your fingertip to smooth any rough spots.
Allow them to dry, uncovered until a crust forms on the surface. About 15-60 minutes or until a dry film develops over the surface of the cookie. For humid areas put a space heater nearby to help dry the cookies faster.
Bake at 300ºF for about 10-15 minutes or until lightly browned. Smaller cookies will bake in 10 minutes, larger cookies will need to bake longer. If not quite baked, bake for an additional 1 minute. Cooled cookies should pull away from the parchment paper without sticking. If they do stick, they were not baked enough.
Let cool fully before removing from the parchment and filling with buttercream. Cookies can be stored in the refrigerator for up to 5 days. Shells can be frozen for 6 months in an airtight container.
For the buttercream
Place egg white, powdered sugar and salt in the bowl of a stand mixer and whip on high for 5 minutes. Then add in softened butter and vanilla and whip until light and creamy.
You can replace the vanilla with any other type of flavoring that you desire