Prepare the Baking Sheets: Line baking sheets with parchment paper or silicone mats.

Sift the Dry Ingredients: Sift the powdered sugar and almond flour together into a large bowl.

Whisk Egg Whites: In a clean, dry bowl, whisk the egg whites and salt with an electric mixer until soft peaks form.

Add Sugar: Gradually add the granulated sugar while whisking continuously until stiff, glossy peaks form.

Add Color: If using, add a few drops of gel food coloring to the meringue and mix until well blended.

Combine Dry and Wet Ingredients: Carefully fold the dry ingredients into the meringue in two additions using a rubber spatula.

Macaronage: Continue folding the batter until it has a lava-like consistency.

Pipe the Shells: Transfer the batter to a piping bag fitted with a round tip. Pipe small circles onto the prepared baking sheets (approx. 1.5 inches wide).

Tap the Sheets: Gently tap the baking sheets on the counter a few times to release any air bubbles.

Rest the Shells: Let the piped shells rest at room temperature for 30 to 60 minutes, until a skin forms on the surface.

Bake the Shells: Bake in a preheated oven at 300°F (150°C) for 12 to 15 minutes.

Cool the Shells: Let the shells cool completely on the baking sheets before removing.

Cream the Butter: In a bowl, beat the softened butter with an electric mixer until light and fluffy.

Add Powdered Sugar: Gradually add the powdered sugar, mixing on low speed.

Add Liquids: Add the milk or heavy cream, vanilla extract, and salt.

Beat Until Smooth: Increase the mixer speed to medium and beat until the buttercream is smooth and creamy.

Fill the Macarons: Transfer the buttercream to a piping bag fitted with a round tip. Pipe a small amount of buttercream onto one shell, and then top with another shell.