For Macarons:

1. Preheat oven to 300ºF
2. Line a baking sheet with a silicon macaron template or parchment paper.
3. Sift together the powdered sugar, salt, and almond flour. Pulse the mixture in a food processor 8-10 times to blend the ingredients together.
4. Whip the egg whites on low to frothy consistency and slowly add the sugar while mixing on low. Once the egg whites turn white and you can see some lines forming on the surface from the whisk, add cream of tartar, whipping on medium until soft, glossy peaks form. Add vanilla and food coloring to the meringue during the soft peak stage. Then, continue whipping on medium-high until you get stiff and shiny peaks that gather and bunch on the inside of the whisk.
5. Add 1/3 of the almond mixture to the meringue. Fold a spatula under the batter and around the edges and then cut through the center until almond flour is mixed in. Continue with the rest of the almond flour and fold until well mixed. Continue folding around the outside edge until the batter forms a ribbon and moves like lava. Your meringue is ready when it forms a ribbon off the spatula and the batter that settles almost dissolves all the way back into the rest of the batter but still leaves a bit of a line.
6. Place parchment paper onto your sheet pan. Pipe small rounds about 1" in diameter. Drop the pan onto the table several times from about 5" above the table to release bubbles. Use a toothpick to remove big pockets of air trapped under the surface.
7. Allow them to dry, uncovered until a crust forms on the surface. About 15-60 minutes or until a dry film develops over the surface of the cookie.
8. Bake at 300ºF for about 10-15 minutes or until lightly browned. If not quite baked, bake for an additional 1 minute.
9. Cooled cookies should pull away from the parchment paper without sticking. Let cool fully before removing from the parchment and filling with buttercream.

For Buttercream:

1. Soften butter, then beat in a stand mixer until fluffy.
2. Add in powdered sugar slowly and continue mixing until combined. Add in vanilla and milk. Mix until smooth. The buttercream should be thick but soft enough to pipe onto macarons.
3. Place buttercream in a piping bag with a round tip, and squeeze onto macarons in a circular motion, covering most of the surface.
4. Carefully place another macaron cookie on top and gently twist to form a sandwich cookie.

Recipe Notes: Cookies can be stored in the refrigerator for up to 5 days. Shells can be frozen for 6 months in an airtight container. These are gluten-free and can be dairy-free by substituting vegan butter and almond milk in the buttercream recipe.