Preheat oven 180 C / Gas 4. Line a cupcake tin with paper cases.
In a bowl combine flour, sugar, cocoa and bicarbonate of soda; mix until well combined. Add water, jam, mayonnaise and vanilla. Beat with an electric mixer until smooth and well combined. Fold in the crushed chocolate.
Spoon mixture into paper cases and bake in preheated oven for 15-20 minutes, until top springs back when lightly pressed. Cool on a wire rack.
For the icing: Beat together cream cheese, butter and cream. In a microwave-safe dish, melt the chocolate until smooth, 20 seconds at a time, stirring between. Beat melted chocolate into butter mixture. Stir in icing sugar, vanilla and cocoa. Refrigerate until cupcakes have cooled and are ready to be decorated.