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

In a separate clean mixing bowl, use a hand mixer or stand mixer to whisk the egg whites. Start on low speed, then increase to medium-high. When soft peaks begin to form, gradually add the granulated sugar and continue to whisk until stiff peaks form.

Add the sifted almond flour and powdered sugar mixture to the whipped egg whites. Fold gently using a spatula. If desired, add food coloring.

Transfer the macaron batter into a piping bag with a round tip. On a baking sheet lined with parchment paper, pipe heart shapes.

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.

In a medium mixing bowl, beat the softened butter until smooth. Gradually add the powdered sugar 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. If desired, add red food coloring.

Once the macaron shells have cooled, pipe a small amount of buttercream onto the flat side of one shell and sandwich with another shell.

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