Prepare the dry ingredients: Sift the powdered sugar and almond flour together into a large bowl to ensure there are no lumps. Set aside.

1. Whisk the egg whites: Beat the egg whites with a pinch of cream of tartar (if using) until soft peaks form. Gradually add granulated sugar, and continue beating until stiff peaks form and the meringue is glossy.
2. Fold the dry ingredients into the meringue: Gently fold the sifted almond flour and powdered sugar mixture into the meringue using the macaronage technique. Add food coloring if desired, and fold until the batter flows like lava when lifted with a spatula.
3. Pipe the macaron shells: Transfer the batter to a piping bag and pipe 1-1.5 inch circles onto a lined baking sheet. Tap the sheet to release air bubbles.
4. Rest the macarons: Let the piped shells sit at room temperature for 30-60 minutes, until they form a skin that’s dry to the touch.
5. Bake the macarons: Preheat the oven to 300°F (150°C). Bake the shells for 15-20 minutes. They should be firm and not sticky. Allow them to cool completely on the baking sheet.
6. Make the filling: Beat the butter with powdered sugar until smooth. Add the vanilla extract and heavy cream or milk until the filling reaches a smooth, spreadable consistency.
7. Assemble the macarons: Once the shells are cooled, carefully remove them from the baking sheet. Pair them up, pipe filling onto the flat side of one shell, and sandwich with the matching shell.
8. Let the macarons mature: Place the filled macarons in an airtight container and refrigerate for at least 24 hours to allow the flavors to meld.
9. Serve: Bring the macarons to room temperature before serving and enjoy the delicate crunch and melt-in-your-mouth filling.