Grease and line the bottom of 3 x 20cm (8 inch) sandwich tins.
Make the sponges
Put the milk chocolate and dark chocolate in a heatproof bowl. Melt in the microwave in 30 second bursts, stirring between each blast and taking care not to overheat the chocolate. When there are only a few solid pieces left, you can stop heating as they will melt from the residual heat. Set aside to cool.
Put the eggs in a large bowl and whisk.
Add the buttermilk, vanilla, butter and sugar. Whisk until everything is combined.
Add the melted chocolate and whisk again.
Sift in the flour and bicarbonate of soda. Fold together,
Immediately, divide the mixture between the three prepared tins and level off.
Put the tins in the oven - you may need to use two shelves. Bake for 25-30 minutes until risen and firm to the touch.
Allow the cakes to cool in their tins for minutes before removing from the tins and transferring to a wire rack to cool completely.
Make the fudge frosting
Put the milk chocolate and dark chocolate in a heatproof bowl and melt in the microwave in the same way as before. Allow to cool until only slightly warm to the touch. This could take up to 20 minutes
Whilst whisking the chocolate, gradually pour in the cream, whisking until thick and glossy.
Assemble the cake
Place the first sponge on a board and top with one third of the fudge frosting, spreading just shy of the edges (it will spread more when the subsequent layers are placed on top).
Put the next cake on top and top with a further a third of the fudge frosting.
Position the final cake on top, then pile on the remainder of the frosting, using your spoon of a palette knife to add attractive swirls and peaks.
Your cake is ready to serve. For neat slices, chill the cake for 30 minutes before serving.