Photograph: Jonathan Gregson
Preheat the oven to 180°C, fan 160°C, gas 4. Melt the chocolate in a heatproof bowl over a pan of barely simmering water, making sure the bowl is not touching the water. When the chocolate is glossy and completely melted, remove from the heat and set aside.
Lightly butter and flour 2 x 23cm sandwich or springform tins (or you can use 3 x 20cm tins for a triple-layer version) and line with nonstick baking paper. Sift the flour and baking powder together and set aside.
Using an electric hand mixer, cream the butter until it is light and pale – this will take about 3 minutes. Add the sugars and cream together until very fluffy. Add the eggs, one at a time, beating well after each addition, then add the vanilla extract. Now incorporate the melted chocolate, mixing until everything is evenly combined.
Finally fold in the flour mixture and milk alternately, using a large metal spoon or spatula. The batter should be smooth and glossy. Divide the mixture between the prepared tins and bake on the middle shelf of the oven for 35 minutes or until a skewer inserted in the centre of the cakes comes out clean.
Place the tins on a wire rack and leave to cool for 5 minutes or so, then turn the cakes out on to the rack and leave to cool completely before icing.
To make the icing, using an electric hand mixer, beat all the ingredients together until soft and completely smooth. Spread over the cakes while still soft – first lay one sponge on a board and spread generously with half of the icing. Place the second sponge on top and spread with the remaining icing. Place in the fridge for an hour or so before serving. Grate a little dark chocolate over the top to serve.
Get ahead: you can make the cake a few days ahead an store in an airtight container; ice a few hours before serving. Can be frozen (uniced).