For the macarons, preheat the oven to 140C/120C Fan/Gas 1. Line 2 baking trays with silicone paper and draw 12 well-spaced circles, each 4.5cm/1¾in in diameter, onto each sheet of paper. Turn the silicone paper over but make sure that you can still see the circular templates.
Blend the icing sugar and ground almonds to a fine powder in a food processor, scraping the mixture from the sides of the bowl halfway through. Set aside.
Dissolve the caster sugar in 50ml/2fl oz water in a saucepan, then bring the mixture to the boil. Continue to boil until the mixture reaches 110C on a sugar thermometer. (Caution: do not leave the pan unattended.)
Meanwhile, whisk the egg whites in a large bowl using an electric whisk set to a high speed, until soft peaks form when the whisk is removed.
Gradually add the sugar syrup to the egg whites in a thin stream, whisking continuously for at least 2 minutes, until the egg-white mixture is smooth and glossy and has cooled slightly.
Transfer half of the meringue to a clean bowl, then sieve the ground almond mixture over it and whisk together to form a thick paste. Gently fold in the remaining meringue – you will end up with a smooth mixture that will be easy to pipe.
Spoon the macaron mixture into a piping bag fitted with a 7mm plain nozzle. Secure the silicone paper to the baking trays by piping a dab of the macaron mixture into each corner.
Pipe dollops of the mixture onto the silicone paper, using the templates - aim to just fill each circle. While piping, hold the bag upright and press down lightly to fill the circle with just enough macaron mixture, then quickly whip away the nozzle so that the macaron mixture settles to create a smooth surface. If the mixture doesn't settle of its own accord, use a wet finger to gently press down any peaks.
Set the macarons aside for at least 30 minutes, until the circles have spread slightly and settled.
Bake the macarons in the oven for 20-25 minutes, or until the shells have a crust on them and have risen slightly. Remove from the oven and set aside to cool on the baking sheet.
For the chocolate mousse, suspend a heatproof bowl over a pan of barely simmering water. (Do not allow the base of the bowl to touch the water.) Add the chocolate and butter and stir together until melted. Remove the bowl from the heat and set aside.
Suspend a separate heatproof bowl over the pan of water and whisk the egg yolks and sugar together until pale and thick. Remove the pan from the heat, keeping the bowl inside it.
Whisk the egg whites in a large bowl using an electric whisk set to a high speed, until soft peaks form when the whisk is removed.
Half-fill 2 large wine glasses or glass dessert bowls with a few macaron shells (crumble them if necessary). Drizzle a little red wine over each.
Whisk the warm chocolate mixture into the egg yolk and sugar mixture until well combined, then lift the bowl from the saucepan and gently fold in the egg whites, a spoonful at a time, until they are totally incorporated into the mousse mixture.
Fill the wine glasses with the mousse mixture and top each with one more macaron shell. Chill in the fridge for 2 hours, until set - or until ready to serve.