Make the macarons: preheat the oven to 160C/ 140C fan.
Blitz the icing sugar and ground almonds in a food processor to a very fine mixture then sieve into a bowl.
In a separate bowl, whisk the egg whites with a pinch of salt to soft peaks, then gradually whisk in the caster sugar until thick and glossy. Add the food colouring.
Fold half the almond and icing sugar mixture into the meringue and mix well. Add the remaining half, making sure you use a spatula to cut and fold the mixture until it is shiny and has a thick, ribbon-like consistency as it falls from the spatula. Don’t mix too much as you still want to be able to pipe the mixture- but only just. Spoon into a piping bag fitted with a 1cm plain nozzle.
Line 3 baking sheets with baking paper. Pipe small rounds of the macaroon mixture, about 3cm across, onto the baking sheets. Give the baking sheets a sharp tap on the work surface to ensure a good ‘foot’. Leave to stand at room temperature for 10-15 minutes to form a slight skin. This is important – you should be able to touch them lightly without any mixture sticking to your finger.
Bake for 15 minutes. Remove from the oven and cool.
Make the buttercream: in a bowl, beat the butter until light and fluffy, then beat in the icing sugar. Next beat in the gel colouring, then spoon into a piping bag and set aside.
Make the icing: sieve the icing sugar into a small bowl. Add a small amount of cold water and mix to a runny coating consistency. Dip a toothpick in the gel food colour and run it through the icing without mixing it in.
To assemble: pair up the macaron halves with ones of a matching size.
Dip one half in the royal icing and let any excess drip off, then leave to dry for a couple of hours.
Pipe the buttercream onto one half of the macarons. Put the other macaron half on top and push together.