Preheat the oven to 180C / 350F (160C / 320F fan).
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.
If any of the cakes have domed on top, you can trim them flat with a sharp, serrated knife.
Make the frosting
Put the icing sugar, milk and butter into a large bowl.
Whisk, starting slowly, then increasing speed until you have a pale, aerated buttercream.
Melt and cool the chocolate and add to the bowl.
Whisk briefly to give a creamy chocolate buttercream
Assemble The Cake
Place the first sponge on a board. Top with one quarter of the buttercream, 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 quarter of the buttercream.
Position the final cake on top, then use the remainder of the chocolate buttercream to cover the top and sides of the cake. You can add a swirl or peaks on top with a palette knife, if you like.
Your cake is ready to serve. For neat slices, chill the cake for 30 minutes before serving.
I decorated the top of my cake with leftover chocolate bars for extra drama!
Notes
Please note that the estimated nutritional calculation does not include any optional toppings.