Put the chocolate and measurement water in a small saucepan and heat gently until the chocolate has melted. Leave to cool.
Beat together the butter, sugar, eggs, flour, cocoa powder, bicarbonate of soda and baking powder in a bowl until smooth and creamy. Add the cooled chocolate and soured cream and stir until evenly combined.
Divide the mixture evenly between paper cupcake cases arranged in a 12-hole cupcake tray. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 25 minutes until risen and just firm to the touch. Transfer to a wire rack to cool.
Make the ganache. Heat the cream and icing sugar in a saucepan until bubbling around the edge. Pour over the chopped chocolate in a heatproof bowl and leave until the chocolate has melted, stirring frequently. Cover and chill until beginning to thicken. Stir in the raspberries and spoon over the cupcakes. Top each cake with a fresh raspberry.