Sift the powdered sugar and almond flour together into a bowl to ensure the mixture is fine and lump-free. Set it aside.

In a separate clean mixing bowl, whisk the egg whites using a mixer, starting on low speed and increasing to medium-high. When soft peaks begin to form, gradually add the granulated sugar and continue whisking until stiff peaks form.

Gently fold the sifted almond flour and powdered sugar mixture into the whipped egg whites using a spatula. Add food coloring (if using) and continue folding until the mixture flows off the spatula like thick lava.

Transfer the macaron batter into a piping bag with a round tip. Pipe heart shapes on a baking sheet lined with parchment paper.

Let the piped macarons sit at room temperature for 30 minutes to 1 hour to form a dry skin.

Preheat the oven to 300°F (150°C). Bake the macarons for 18 to 20 minutes or until firm to the touch.

Beat the softened butter in a medium mixing bowl until smooth.

Gradually add powdered sugar, mixing well, followed by vanilla extract and a pinch of salt.

Add heavy cream or milk one tablespoon at a time until a smooth, spreadable consistency is reached. Add food coloring (if using).

Once cooled, flip the macaron shells, pipe buttercream on the flat side of one, and sandwich with another shell.

Refrigerate assembled macarons for at least 24 hours for best flavor and texture.