Draw 2” circles approximately 2 inches apart on parchment paper.
Line baking sheets with parchment paper (written side down).
Sift powdered sugar into medium-sized bowl.
Add almond flour to powdered sugar, using fingers to break up clumps.*
Whisk sugar and flour in a small bowl to combine.
Whisk egg whites in bowl of stand mixer until foamy.
Add sugar and continue whisking until firm peaks are formed.
Add salt, food coloring, and ½ tsp lemon extract, and whisk for 10 seconds.
Add powdered sugar mixture to egg whites in 4 parts, folding between each.
Using a small, plain nozzle, pipe cookie batter into circles.
Bang trays on counter to remove air, and allow to sit for 30 minutes before baking.**
Preheat oven to 300.
Bake for 16-18 minutes, rotating trays halfway through baking.
Allow to cool on baking sheets for 10 minutes.
Cream butter in bowl of stand mixer, using paddle attachment.
Turn mixer off, add powdered sugar, lemon juice, zest, and extract, and turn to low speed to combine.
Add milk, and mix at speed 2 until combined.
Remove cookies from parchment paper and match like-sized cookies.
Transfer buttercream to pastry bag fitted with small, plain nozzle.
Pipe a small amount of frosting to bottom of one cookie in each pair; top with other cookie.
To allow frosting to set up, place finished macarons in the refrigerator for 10-15 minutes.