01 - Line baking sheets with parchment paper or silicone macaron mats. Fit a piping bag with a round tip approximately ½ inch in diameter.
02 - Sift almond flour and powdered sugar together into a large bowl. Discard any coarse particles that do not pass through the sieve.
03 - In a clean, grease-free bowl, beat egg whites until foamy. Add cream of tartar. Gradually incorporate granulated sugar while beating until stiff, glossy peaks form.
04 - Gently fold the sifted dry mixture into the meringue in three additions. Using a spatula, fold and press batter against bowl until it flows in thick ribbons and a figure-8 motion holds without breaking.
05 - If using gel food coloring, add now and fold until evenly distributed.
06 - Transfer batter to prepared piping bag. Pipe circles 1½ inches in diameter onto baking sheets, spacing shells 1½ inches apart.
07 - Firmly tap baking sheets on counter several times to eliminate trapped air. Use a toothpick to pop any remaining visible bubbles.
08 - Let piped shells rest uncovered at room temperature for 30-60 minutes until a dry skin forms and batter does not stick to a light finger touch.
09 - Preheat oven to 300°F. Bake one tray at a time for 12-15 minutes, rotating tray halfway through. Shells are complete when feet have formed and they lift cleanly from parchment.
10 - Allow shells to cool completely on baking sheets before removing.
11 - Beat softened butter until creamy. Add powdered sugar, vanilla, salt, and 1 tablespoon milk. Whip until fluffy, adding additional milk as needed for smooth, pipeable consistency.
12 - Pair shells by similar size. Pipe a dollop of buttercream onto flat side of half the shells. Gently press remaining shells on top to create sandwiches.
13 - Refrigerate assembled macarons in an airtight container for 24 hours for optimal texture. Bring to room temperature before serving.