Photograph: Maja Smend
Line 2 large baking trays with baking paper. Draw 12 x 6cm diameter circles on each sheet of paper, then turn the sheets over.
For the coffee macaroon cookies, beat the egg whites to stiff peaks using a hand-held electric whisk, then add the caster sugar gradually with a few drops of the food colouring and the coffee extract. Using a large rubber spatula, fold the ground almonds and icing sugar into the whisked whites, until you have a glossy mixture.
Transfer the mixture into a piping bag fitted with a 1cm plain nozzle. Pipe 12 x 6 cm diameter macaroons on to the circles drawn on one of the lined trays. Sprinkle a little crushed sea salt on top of each, then set aside for 30 minutes at room temperature – this enables a crust to form.
For the chocolate macaroon cookies, whisk the egg whites and add the caster sugar gradually. Fold the ground almonds, icing sugar and cocoa into the whisked whites. Transfer the mixture to a piping bag and pipe the macaroons, as before, so you have 24 macaroons in total. Set the chocolate macaroon cookies aside for 30 minutes for a crust to form. Preheat the oven to 180°C, fan 160°C, gas 4.
Bake the macaroon cookies for 15 minutes. Leave to cool on the baking trays. Meanwhile, for the filling, cut 25g of the butter into small pieces. Heat the sugar in a heavy-based pan over a medium heat until it turns a dark caramel colour; stir from time to time. Take the pan off the heat and stir in the pieces of butter.
In a separate pan, heat up the cream. Put the caramel back on the heat and gradually add the hot cream; allow to bubble for a few minutes.
Pour the mixture into a bowl and leave to cool to room temperature.
Whisk the remaining butter till pale and fluffy, then gradually whisk into the cooled salted caramel.
Pipe a blob of the salted caramel filling on to 12 of the macaroon cookie halves, then sandwich with another macaroon.
Chill the macaroon cookies for at least 6 hours before eating.
Get ahead: make a day ahead; store in the fridge.
Kitchen secret: chilling macaroon cookies allows the filling to set and the shell to soften slightly.