Preheat the oven to 320F or 300F for a fan oven.
Sieve icing sugar before the ground almonds, into a bowl and mix together carefully.
In a separate bowl, whisk the salt and egg whites until soft. Add the fine sugar a little at a time. Continue to whisk until glossy and thick. Stir in the sugar / almond mix.
Use a piping bag with a 1cm nozzle and fill with macaron mixture. Place a silicon mat onto a baking sheet. Use the piping bag to form small blobs on the sheet.
Tap the side of the baking sheet to help the mixture lose air bubbles. Leave to dry for 20 mins. It will become shiny.
Bake macarons for 7-8 mins then open the oven to release steam. Close and cook for another 7-8 mins. They are fully cooked when they are firm.
Slide mat onto a wire rack to cool completely. They break easily if not completely cooled.
Make the filling:
Take the softened butter and beat it until it is fluffy, then add the icing sugar and continue to beat it. Beat in any other flavorings you choose here.
Place 1/2 tsp of filling on flat side of one macaron, and place it together with another like a sandwich. Twist slightly to bond the two.
Macarons can be eaten immediately but are best after put in the refrigerator for 24 hours.