01 - Line baking sheets with parchment paper or silicone macaron mats. Fit a large piping bag with a round tip approximately ½ inch in diameter.
02 - Sift almond flour and powdered sugar together into a large bowl, discarding any coarse bits that remain in the sieve.
03 - In a clean, grease-free bowl, beat egg whites until foamy. Add cream of tartar and gradually incorporate granulated sugar while beating. Continue until stiff, glossy peaks form.
04 - Gently fold the sifted almond flour mixture into the meringue in three additions. Use a spatula to fold and press the batter against the bowl until it flows in thick ribbons and you can draw a figure 8 without it breaking.
05 - If desired, add gel food coloring and fold until evenly distributed throughout the batter.
06 - Transfer batter to the prepared piping bag. Pipe circles 1 ½ to 2 inches in diameter onto the baking sheets, leaving about 1 ½ inches of space between each shell.
07 - Firmly tap the baking sheets against the counter several times to release trapped air bubbles. Use a toothpick to pop any visible bubbles for smooth tops.
08 - Let the piped shells rest uncovered at room temperature for 30 to 60 minutes, or until a skin forms and the shells no longer stick to your finger when lightly touched.
09 - Preheat oven to 300°F. Bake one tray at a time for 12 to 15 minutes, rotating the pan halfway through. The shells are done when they have formed feet and lift cleanly off the parchment paper.
10 - Allow the shells to cool completely on the baking sheets before removing them.
11 - Beat the softened butter until creamy. Add powdered sugar, vanilla extract, salt, and 1 tablespoon of milk. Beat until fluffy and smooth, adding more milk as needed to achieve a pipeable consistency.
12 - Pair similar-sized shells. Pipe a dollop of buttercream onto the flat side of half the shells. Gently press the remaining shells on top to create sandwiches.
13 - Refrigerate the assembled macarons in an airtight container for 24 hours before serving. Bring to room temperature before enjoying.