Begin by sifting the powdered sugar and almond flour together in a bowl, eliminating any lumps for a smooth shell texture.

In a separate bowl, beat the egg whites on medium speed until frothy. Gradually add granulated sugar, increasing the speed to high until stiff peaks form.

Gently fold the sifted dry ingredients into the whisked egg whites using a spatula. If desired, add food coloring.

Prepare a piping bag fitted with a round tip and fill it with the macaron batter. Pipe heart-shaped shells about 1.5 inches wide.

Allow the piped macarons to rest at room temperature for 30 minutes to 1 hour.

Preheat the oven to 300°F (150°C). Bake the rested macarons for 18-20 minutes, rotating the baking sheet halfway.

While macarons cool, prepare the buttercream filling by beating unsalted butter until creamy and mixing in powdered sugar, vanilla extract, and heavy cream.

Pair the cooled macaron shells by size and pipe a dollop of buttercream onto one shell before sandwiching it with another.

For the best flavor and texture, refrigerate the assembled macarons for at least 24 hours before serving.