Optional: Place the egg whites in a loosely covered bowl and refrigerate them for 3 days to slightly dehydrate them.

Bring the aged egg whites to room temperature for about 30 min before mixing.

Process the almond flour briefly to make it finer, taking care not to turn it into almond butter.

Sift the processed almond flour twice, discarding any coarse pieces that do not pass through the sieve.

Sift the powdered sugar with the almond flour into a clean bowl, then set the dry mixture aside.

Add 100 g egg whites to a clean, grease-free mixing bowl and whisk until they become foamy.

Add the cream of tartar to the foamy egg whites, then continue whisking.

Slowly add the 90 g granulated sugar in small additions of about 1/8 cup while continuously whisking.

Add the vanilla extract, salt, and desired food coloring to the meringue.

Whisk until the meringue forms thick, stiff peaks and does not move when the bowl is inverted.

Fold one-third of the sifted almond flour mixture into the meringue gently with a rubber spatula.

Fold in the remaining dry mixture in two additions, adding each portion when the preceding one is barely incorporated.

Continue folding the batter until it flows like lava and a ribbon can form a figure eight without breaking.

Fit a piping bag with a large round piping tip, then transfer the macaron batter into the bag.

Line two baking sheets with parchment paper and pipe evenly sized rounds of batter, spacing them about 1 inch apart.

Firmly tap each baking sheet on the counter several times to release trapped air bubbles.

Leave the piped shells uncovered at room temperature for about 40 min until a dry skin forms on their surface.

Preheat the oven to 300°F (150°C) while the macaron shells rest.

Bake the shells at 300°F (150°C) for 12 to 14 min, rotating the baking sheets halfway through baking.

Cool the baked shells on their baking sheets for at least 10 min before removing them.

Transfer the shells carefully to a wire cooling rack and cool them completely.

Pipe or spread frosting onto the flat side of half the cooled shells.

Place a remaining shell over each filled shell and gently press to make macaron sandwiches.