Break the chocolate into smaller chunks and melt it very slowly over a water bath. Once the chocolate has melted, take the bowl off the water bath and place it on a kitchen towel to cool down a little.
Open your tin of chickpeas and drain the chickpea water into large glass bowl. The bowl and the utensils you’ll use to whip aquafaba need to be completely grease-free as even the tiniest grease residue can prevent aquafaba from reaching stiff peaks. One 400 g tin should give you 240 ml / 1 cup of chickpea water. Reserve the chickpeas for another use, like a salad or burgers.
Using an electric egg whisk, whip the chickpea water into stiff peaks – this is super important. The best way to check if you have achieved stiff peaks is to invert the bowl with whipped aquafaba. If aquafaba slides down even a tiny bit, you are not there yet – keep on whipping until it does not move at all. The good news is that (as opposed to egg whites) aquafaba cannot be overwhipped.
Check that the chocolate has cooled down sufficiently – it should not be warm to the touch or else chocolate may seize and you will end up with grainy mousse.
Gently fold melted chocolate into whipped aquafaba using a silicone spatula. Do not worry if your fluffy mixture deflates once the chocolate has been added – that’s perfectly normal. By the time you’ve added all of your melted chocolate, the mousse MAY have more of a pourable consistency.
Divide the mixture between 4 glasses. Gently stir the mixture between putting it into individual glasses as a bit of melted chocolate tends to drop to the bottom sometimes.
Put glasses into the fridge for minimum 3 hours for the mousse to set.