Line a baking sheet with parchment paper, and grease with butter or non-stick spray.
Beat egg whites until frothy.
Add salt and cream of tartar.
Beat until stiff peaks form.
Continue to beat, and slowly add the caster/granulated sugar one tablespoon at a time.
You can divide the mixture into separate bowls, and add the optional colour to the egg mixture.
Fold in the almond flour.
Put the mixture into a piping bag, and pipe out 4 cm circles. at least 5 cm apart.
Dip a finger into water, or use a wet silicone spatula to gently smooth out the tops.
Set macarons aside, uncovered for 1 hour, to let them form a thin crust.
Bake at 310F for 12-15 minutes.
Let cool completely, then make cookie sandwiches with your favourite filling - something like a vanilla buttercream, Nutella, cream cheese frosting, chocolate ganache, etc.
