Chop the chocolate into small pieces. Put equal parts of cold water and ice into a large plastic/steel bowl and mix.
Put 150ml boiling water into a second bowl which has to fit into the larger bowl with the ice water.
Now add the chocolate pieces to the boiling water and mix until melted.
Then place the hot water and chocolate mixture over the ice water and whisk continuously for a few minutes until you have achieved a mousse texture. Fill into small ramekins or put into a sealed tub in the fridge.