Get out a good heavy saucepan and snip each marshmallow in half and drop the pieces in the pan.

Add the butter. Chop the chocolate into smallish pieces.

Add water from the kettle, and put the pan on the heat, stirring occasionally until everything is smooth and melted.

Remove from the heat. Whip the cream with vanilla extract until thick, then fold into the chocolate mixture until totally absorbed and smooth again.

Pour into 6 small ramekins or glasses, and cover with cling-film.

Put into the fridge to set, this will take at least 3 hours.