Preheat the oven to 160C/140C fan/gas mark 3.

Grease a 25cm x 12cm x 12cm (2lb) loaf tin and line with baking parchment.

For the cake, put 200g 70% dark chocolate in a bowl with 200g softened unsalted butter and melt it over a pan of barely simmering water, stirring every so often. Remove from the heat and leave to cool for 5 minutes.

Meanwhile, in a large mixing bowl combine 85g self-raising flour, 85g plain flour and ¼ tsp bicarbonate of soda with 150g light muscovado sugar, 150g golden caster sugar and 30g sifted cocoa powder. Gently pour in 3 medium beaten eggs and 75ml buttermilk, and carefully beat to incorporate.

Stir through the butter and chocolate mix until smooth. Pour into the prepared cake tin two-thirds of the way up the sides. If there is any mixture left over you can bake it in some muffin moulds.

Bake for about 1 hour 30 minutes until a skewer in the centre of the cake comes out clean.

Remove from the oven and leave to cool for about 15 minutes then run a knife around the edge and carefully turn out on to a cooling rack. Leave to cool completely.

Meanwhile make the ganache topping. Pour 300ml double cream into a saucepan and add 60g caster sugar. Gently bring to a simmer and heat until the sugar has dissolved. Put 200g broken 70% chocolate into a heatproof bowl, pour over the cream mixture and beat until the chocolate has melted and you have a smooth and silky ganache. Set aside.

Carefully cut the cake lengthways into four. Spread each slice of cake with some of the chocolate ganache then sandwich it back together.

Spread the rest of the mixture on the top and down the sides of the cake in a swirly motion to create a nice effect. You may need to heat the chocolate mixture up a little to make it easier to spread.

Dust about 20 walnut pieces, lightly toasted, with icing sugar and arrange on top. Transfer to a plate or board and serve.