Melt the chocolate in a heatproof bowl set over a pan of barely simmering water. Once melted, remove from the heat.
Whisk the egg whites to soft peaks, add the sugar and whisk until combined.
Dissolve the instant coffee in 2-3 tsp boiling water and stir into the chocolate along with the egg yolks. Whisk in a third of the egg white mixture, then gently fold in the rest until just combined. Divide among six espresso cups and chill in the fridge for 4 hours, or until set.
Preheat the oven to 180°C, gas mark 4. Tip the nuts onto a baking tray and roast for 5-8 minutes, until golden. Crush with a rolling pin and tip into the milk. Leave to infuse for 30 minutes.
Pass the milk through a sieve into a saucepan set over a medium-high heat. Bring to the boil and remove from the heat. Whisk with a milk frother until thick and spoon over each mousse. Dust with cocoa powder and serve.