Preheat the oven to 155'C.
Sift the ground almonds and the icing sugar together.
Whisk the egg whites until white and fluffy and then add the caster sugar to the the egg whites and continue whisking until you have stiff, glossy peaks.
Fold the ground almonds and icing sugar into the meringue mixture.
Cut and fold the mixture with a spatula until all of the dry ingredients are incorporated and the mixture is smooth, shiny and has reached the ribbon stage.
Carefully add the food colouring making sure not to overwork the mixture. Spoon the mixture into a piping bag.
Pipe 2cm circles in straight lines across a baking sheet lined with a silicon paper. Leave a 1 cm gap between each macaroon shell. Drop the tray of macaroons onto the work surface from about 30 cm to remove any large air bubbles.
Allow the macaroons to rest at room temperature for 20 minutes or until the macaroon shells are no longer sticky to touch.
Bake in the oven for 15 minutes. If you are baking lots of macaroons you may need to keep the oven door slightly ajar (with a spoon or paper) to prevent steam building up in the oven.After 15 minutes, check the macaroons are cooked by gently lifting a macaroon at the edge of a tray. If the test macaroon does not stick, allow the tray of macaroons to finish cooking on the tray as it cools.
Allow the macaroons to cool completely before filling.
Place the remaining 170g sugar in a heavy bottomed pan and allow to melt without stirring.
Once you have a golden caramel, add a knob of butter and the cream. Mix well then remove from the pan and allow to cool.
Place the caramel in a food processor, add the salt and mix the remaining butter in a little at a time until you have a smooth, creamy filling.
Cover half of the macaroon shells with the caramel filling and then top with a second macaroon shell.
«Oven temperatures can vary quite dramatically. It is safest to bake macaroons at a lower temperature and for longer if necessary to keep them soft and chewy in the middle.»