Step 1: Sift almond flour and powdered sugar together into a bowl, discarding any large pieces.
Step 2: In a clean, dry bowl, whip egg whites with cream of tartar until foamy, then gradually add granulated sugar and whip to stiff, glossy peaks.
Step 3: Fold the dry ingredients into the meringue in two additions, mixing until the batter flows like lava and forms a ribbon.
Step 4: Pipe 1.5-inch circles onto parchment-lined baking sheets, tap sheets firmly to release air bubbles, and let rest for 30-60 minutes until skins form.
Step 5: Preheat oven to 300°F (150°C) and bake for 15-18 minutes, rotating halfway, until feet are set and tops are firm.
Step 6: For the buttercream, beat butter until creamy, add powdered sugar and vanilla, then thin with cream to a pipeable consistency.
Step 7: Cool macaron shells completely, then pipe buttercream onto half and sandwich with another shell.
Step 8: Refrigerate in an airtight container for 24 hours to mature before serving for best texture.