In a food processor, combine almond flour and powdered sugar and blend to obtain a fine powder. Sift into a bowl. Repeat sifting. In a mixing bowl with an electric beater, whisk egg whites until foamy (about 1 minute). Add a pinch of cream of tartar and salt. Whisk until meringue begins to thicken. Slowly add fine granulated sugar, a little at a time. Whisk on high until egg whites have the texture of shaving foam with soft peaks. Add sifted almond mixture to the egg whites. Using a silicon or plastic spatula, gently fold the mixture until the almond powder and egg whites are fully incorporated. Mixture is done when it is smooth, shiny, and slightly runny (like a cake batter). Do not mix beyond this point. If using a coloring or flavor, it must be added at this point. Place the mixture in a piping bag with about a ¼” tip. Pipe about a nickel-size of the mixture on a baking sheet lined with parchment paper or silicone pad, about 1 inch apart. Important: set baking sheet on the counter for about 30 minutes at room temperature until a slight crust develops over the macarons. Bake at 325ºF (use standard bake element, not convection) for about 15 minutes. Allow the baking sheets to cool and the macarons to harden before lifting them away from the baking sheet. When cool, fill with icing of your choice, like butter cream icing or ganache.