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 hand mixer or stand mixer, starting on low speed and then increasing to medium-high. When soft peaks begin to form, gradually add the granulated sugar and continue to whisk until stiff peaks form.

Fold the sifted almond flour and powdered sugar mixture into the whipped egg whites gently using a spatula. Add a few drops of food coloring (optional) 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 until firm to the touch.

In a medium mixing bowl, beat the softened butter until smooth and creamy.

Gradually add the powdered sugar, one tablespoon at a time, until fully combined. Stir in the vanilla extract and a pinch of salt.

Add the heavy cream or milk, one tablespoon at a time, to reach a smooth, spreadable consistency. Add a few drops of red food coloring, if desired.

Once cool, flip one macaron shell over, pipe a small amount of buttercream onto the flat side, and sandwich it with another shell.

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