Place the chocolate and 120 ml warm water in a large heatproof bowl, and place it over a saucepan of barely simmering water, making sure the bowl doesn't touch the water.
Allow the chocolate to melt slowly, then remove it from the heat and give it a good stir until it's smooth and glossy. Leave to cool for 5 mins, before stirring in the egg yolks.
In a clean bowl, whisk the egg whites to the soft-peak stage, then whisk in the sugar, about a third at a time, whisking until the whites are glossy.
Using a metal spoon, fold a tablespoon of the egg whites into the chocolate mixture to loosen it, then carefully fold in the rest. You need to have patience here – it needs gentle folding to retain the air, which makes the mousse light
Divide the mousse between the ramekins or glasses and chill for at least 2 hours, covered with clingfilm. Serve with Joe & Seph's popcorn slab.