Place the prunes in a pan with the measured water and bring to the boil. Immediately remove from the heat, transfer to a food processor and whiz until completely smooth.
Return the prunes to the pan with the chocolate pieces and heat over a very gentle heat, stirring continuously until the chocolate has melted. Remove from the heat and beat in the yogurt. Allow to cool.
Divide the dessert between 4 serving glasses and decorate with the chocolate shavings. Chill for about 30 minutes and serve.