Sift together almond flour and powdered sugar into a large mixing bowl. Set aside.

In a clean mixing bowl, beat egg whites and cream of tartar until frothy. Gradually add granulated sugar and whip to stiff peaks.

Fold the sifted flour/sugar mixture into the egg whites carefully until smooth. Divide and tint one part with red gel coloring.

Pipe small circles of the batter onto a lined baking sheet, tap gently to release air bubbles, and let rest for 30-45 minutes.

Preheat the oven to 300°F (150°C) and bake for 15 minutes until shells have a sheen and lift cleanly from the mat.

Beat the softened butter until creamy, gradually mix in powdered sugar, milk, and vanilla until fluffy for the filling.

Pair cooled macaron shells by size, pipe buttercream filling on half, and top with the remaining shells.