(click on the text to highlight each stage):

1. Preheat the oven to 180°C/fan160°C/gas 4. Grease a 20cm round, deep cake tin and line with baking paper. Place the 125g of plain chocolate with 50g of the butter in a bowl and melt over a pan of hot water. Remove from the heat and stir in the remaining butter.
2. Mix the milk and vinegar. Place all the other cake ingredients in a large mixing bowl. Pour in the milk mixture and chocolate butter mixture. Beat until smooth. Spread into the cake tin and bake for 1 hour, until firm in the centre. Cool for 10 minutes, then turn out and cool completely.
3. Meanwhile, make the icing. Melt the 225g of plain broken up chocolate and butter together in the microwave for 3 minutes on medium, stirring halfway through. Alternatively, melt the chocolate and butter in a bowl over a pan of just simmering water. Stir until smooth, then beat in the cream. Cool slightly
4. Halve the cake through the middle. Spread a quarter of the icing on 1 half and sandwich together. Spread the rest over the top and sides and decorate with chocolate curls, silver balls, chocolate buttons or whatever takes your fancy!