Classic French Macaron with Vanilla Buttercream Filling

Name: Classic French Macaron with Vanilla Buttercream Filling
Ingredients: French Macarons; 1 1/4 cups Almond Flour (Blanched); Pinch Salt; 3/4 cup Confectioners' Sugar (packed); 1 tablespoon Powdered Egg Whites; 3/4 cup Granulated Sugar; 1/2 teaspoon Cream of Tartar; 1/2 cup Aged Egg Whites (from 4 eggs, aged covered in the fridge for 1-2 days) - Room Temperature; 4 drops Gel food color (optional); Vanilla Buttercream Filling; 1 Vanilla Bean; 1 cup Granulated Sugar; 3 Egg Whites; 1 pinch Cream of Tartar; 1/4 teaspoon Fine Sea Salt; 2 Sticks, cold unsalted butter, cubed; 1 tablespoon Vanilla Extract