Sieve the icing sugar and ground almonds (if substituting almond flour, use 3/4 cup almond flour) into a large mixing bowl. Throw any almond lumps left behind away.
In a separate, clean bowl whisk the egg whites (add in food coloring now if you're going to add it) and salt until they form soft peaks. Add the caster / fine sugar, a little at a time and continue to whisk until the whites are very thick and glossy (ideally, you should be able to hold the bowl upside down without the whites falling out - go on, I dare you!) Gently stir in the icing sugar and almond mix. The mixture will lose some air and become quite loose, don't worry, this is the way it should be.
Using a piping bag with a 1cm / 1/3" nozzle, fill with the macaron mixture. Place the silicon mat (see note in the introduction) or paper template onto a baking sheet. Pipe small blobs onto the sheet remembering that less is more at this stage because the mixture will settle and form into the allotted spaces.
Gently tap the baking sheet a few times on the work surface to help the macaron mixture to settle and to break any air bubbles, then leave to dry for 20 minutes - the surface of the macaron will become smooth and shiny
Bake the macarons in the preheated for 7 - 8 minutes open the door to release any steam, close the oven door and cook for a further 7 - 8 minutes. The macarons are cooked when they feel firm and are slightly risen.
Slide the mat or greaseproof paper onto a wire cooling rack and leave to cool thoroughly. Do not be tempted to remove the macarons from the mat until they are cold or you will break them.
Make the Buttercream Filling
Beat the softened butter until it is fluffy, then gradually beat in the icing sugar. At this point you can add in any flavorings, you may choose, such as vanilla, raspberry, strawberry, blueberry, pistachio, lime, or lemon. Try to match the flavor to the color of the macaron so people don't get the shock of a pink pistachio.
Place approx 1/2 a tsp of the filling to the flat side of one macaroon and sandwich together with another then twist ever so slightly to create a bond. Continue with the remaining macarons.
The macarons can be eaten immediately but will benefit from being refrigerated for 24 hours (that's if you can resist them for that long) as this will make them even more chewy and tasty.
