Chop the chocolate or cut into very small pieces.
Place the chocolate, butter and marshmallows in a thick bottomed pan. Pour in the hot water.
Set the pan on the stove over very, very low heat to melt the chocolate, butter and marshmallows. Stir often, scraping the bottom to make sure that no chocolate sticks.
When the mixture is smooth, turn off the heat and let it cool for a bit.
While the chocolate mixture cools, whip the cup of cream.
Fold the cream into the chocolate mixture until smooth. Pour the mousse into dessert glasses (I used champagne glasses) and chill in the fridge for about an hour or until the top can be touched without leaving a trace on the finger.
Top the eggless instant chocolate mousse with whipped cream, sprinkle with chocolate bits and serve.