Preheat oven to 160 C / Gas 3. Place 12 paper cases in a bun tin.
Melt the butter, chocolate, sugar and hot water together in a large pan on a low heat, stirring occasionally, then set aside to cool slightly.
Stir the eggs and vanilla into the chocolate mixture.
Sift the flour in to a mixing bowl then stir in the chocolate mixture until smooth. Spoon mixture into the paper cases.
Bake for 20 minutes until firm. Cool the tin for 5 minutes then place on a wire rack.
For the icing, beat the butter in a large bowl until soft. Sift in half of the icing sugar and beat until smooth. Sift the remaining icing sugar and cocoa powder and stir until combined. Add 1/2 of the milk and beat until creamy. Add more milk only if needed.
Spoon icing on top of each cake and decorate with a fresh raspberry.