Preheat the oven to 160°C. Grease the tin and line the base with baking parchment and put aside. For the fudge ganache, put the chocolate, muscovado sugar, butter, evaporated milk and vanilla extract into a heavy-based saucepan and heat gently, stirring constantly, until everything has melted. Pour into a bowl and leave to cool. Once cool, cover and place in the fridge for at least 1 hour or until spreadable. For the cake, mix together the flours, xanthan gum, bicarbonate of soda, sugars and cocoa powder. Melt the chocolate, butter and coffee mixture over a medium heat in a heavy-based saucepan until everything has just melted. Beat the eggs in a separate bowl and add in the yogurt. Add the melted chocolate mixture and the beaten egg mixture into the flour mixture and, using a big metal spoon, stir until everything is well blended and you have a smooth, runny consistency. Pour the mixture into your prepared cake tin and bake in the oven for 1¼ to 1½ hours. Leave to cool in the tin for 30 minutes, then remove from the tin and cool completely on a metal cooling rack. When the cake has cooled, cut it in half horizontally, not worrying if the top cracks because this will be covered by the ganache. Sandwich the two halves together with a third of the ganache, then spread the remaining ganache over the top and sides of the cake, swirling it to give a frosted appearance. Decorate with the white chocolate and serve.