1. Heat oven to 190C/170C fan/gas 5. Butter the base and sides of two 20cm round sandwich tins, then line the bases with baking parchment.
2. In a large bowl, beat all the cake ingredients and a pinch of salt together until pale. Divide the mixture between the prepared tins. Bake for 20 mins or until a skewer inserted into the centre of the cake comes out clean. Leave to cool in the tin for 10 mins, then turn out onto a wire rack to cool completely.
3. For the buttercream, put the chocolate in a heatproof bowl and melt in the microwave, stirring every 30 secs. Leave the melted chocolate to cool for 5 mins. Mash the butter and icing sugar together with a fork, then switch to a wooden spoon or electric beaters, if you have them. Sift in the cocoa with a pinch of salt and pour in the melted chocolate and milk. Mix again until smooth.
4. On a cake stand or large plate, sandwich the cakes together with half of the buttercream, then spread the rest on top. Decorate with chocolate shards, if you like.
5. To make chocolate shards: melt 50g dark chocolate and pour it onto a tray lined with baking parchment or foil. Now melt 25g milk chocolate and 25g white chocolate and drizzle them over the dark chocolate before it sets. Shake the tray gently to level the mixture then leave to set somewhere cool. Chop into shards.