Preheat the oven to 170C/335F/Gas 3. Grease and line two 20cm/8in sandwich tins.
Melt the chocolate, butter and water together in a saucepan over a low heat, stirring occasionally, until smooth. Set aside to cool slightly.
Meanwhile, sift the flours and cocoa powder into a bowl and mix in the muscovado and caster sugar.
Beat the eggs and crème fraîche together in a separate bowl, then beat in the chocolate mixture. Fold in the flour mixture until smooth.
Divide the cake batter between the tins and bake for 35-40 minutes, or until the top is firm to the touch.
Remove the cakes from the oven and set aside to cool completely.
Meanwhile, for the fudge buttercream icing, melt the chocolate in a bowl set over a pan of simmering water. (Do not allow the base of the bowl to touch the surface of the water.)
Beat the butter in a bowl until soft, then gradually beat in the icing sugar. Add the vanilla extract and fold in the melted chocolate until smooth (add a few drops of milk if the mixture is a little stiff).
For the chocolate ganache, heat the cream, vanilla extract, butter and chocolate in a heavy-based pan. Remove the pan from the heat and whisk the mixture until smooth and thickened.
To assemble the cake, run a round-bladed knife around the inside of the cake tins to loosen the cake. Carefully remove the cakes from the tins.
Spread the fudge buttercream over the top of one of the chocolate cakes, then carefully top with the other cake.
Transfer the cake to a serving plate and spoon the chocolate ganache over the top of the cake, allowing it to drizzle over the edge. Garnish with chocolate shavings and serve in slices.