Sift the powdered sugar and almond flour together. In a clean bowl, whip the egg whites with cream of tartar until foamy, then gradually add the sugar and whip to stiff glossy peaks.
Fold the dry ingredients into the meringue with the vanilla until the batter flows slowly from the spatula and the surface smooths within 30 seconds.
Pipe and Bake:
Pipe 1 1/2-inch rounds onto lined baking sheets. Tap the sheets firmly to release air bubbles and rest the shells for 30 to 45 minutes, until dry to the touch.
Heat the oven to 300 F. Bake one sheet at a time for 13 to 14 minutes, rotating halfway through. Cool completely before removing from the paper.
Fill:
Beat the butter until pale. Gradually add the powdered sugar, cream, and vanilla, then beat until smooth.
Pipe buttercream onto half the shells and sandwich with the remaining shells.
Notes
Weigh ingredients for consistent shells.
Rest piped shells until the surface is dry.
Chill filled macarons overnight for the best texture.