For the macaron shells: Sift the powdered sugar and almond flour together to remove lumps. In a clean bowl, beat egg whites until foamy. Add cream of tartar (if using) and beat until soft peaks form. Gradually add granulated sugar, one tablespoon at a time, until stiff peaks form and the meringue is glossy. If using, add gel food coloring at the end.

1. Gently fold the sifted dry ingredients into the meringue. Mix until the batter flows like lava and forms a ribbon when the spatula is lifted.
2. Pipe small, even circles (about 1.5 inches) on a parchment-lined baking sheet. Tap the sheet to release air bubbles. Let the piped macarons rest for 30-60 minutes until the tops are dry to the touch.
3. Preheat the oven to 300°F (150°C). Bake the macarons for 15-18 minutes, or until they easily lift off the parchment paper. Let them cool completely on the baking sheet before transferring to a wire rack.
4. For the filling: For buttercream, beat softened butter until creamy, then add powdered sugar, vanilla, and heavy cream. For ganache, heat heavy cream in a saucepan, pour over chopped chocolate, and stir until smooth. Let the ganache cool to room temperature.
5. Match similar-sized shells. Pipe a small amount of buttercream or ganache on one shell and sandwich it with the matching shell.
6. Refrigerate the assembled macarons for 24 hours to allow the flavors to meld. Serve at room temperature.