Put the marshmallows, butter, chocolate and water in a heavy-based saucepan.

Put the saucepan on the hob, over heat, though keep it fairly gentle, to melt the contents, stirring every now and again. Remove from heat.

Meanwhile, whip the cream with the vanilla extract until thick, and then fold into the cooling chocolate mixture until you have a smooth, cohesive mixture.

Pour or scrape into 4 glasses or ramekins, about 175ml each in capacity, and chill until you want to eat. The sooner the better!