Preheat the oven to gas 4, 180°C, fan 160°C. Butter 2 x 20cm (8in) loose-bottomed cake tins and line with nonstick baking paper.
Put the butter and sugars in a large bowl and beat until smooth. Sift together the flour, bicarbonate of soda and cocoa powder into a separate bowl. Beat the eggs into the butter and sugar mix, one at a time, following each with a spoonful of the flour mixture.
Put the chocolate, treacle and almond or vanilla extract in a medium bowl, set it over a pan of gently simmering water and leave to melt. Remove from the heat when melted and set aside to cool slightly. Then carefully fold into the cake mix.
Fold in the remaining flour, alternating with the warmed milk until smooth. Divide evenly between the two tins, smooth the surfaces of each and then bake on the middle shelf of the oven for 20 minutes or until a skewer comes out clean when inserted into the middle. Turn out onto wire racks and leave to cool completely.
To make the white chocolate icing, put the white chocolate and butter in a large bowl over a pan of gently simmering water. Add 2 tablespoons of boiling water and stir gently until melted and smooth. Remove from the heat and beat in the icing sugar.
For the white chocolate curls, melt the white chocolate over a pan of gently simmering water. Pour onto a plastic chopping board and spread until it forms an even layer about 5mm thick. Place in the fridge and chill for about 15 minutes, until just set. Then run a vegetable peeler over the surface to make thick curls of chocolate.
Put one cake, bottom side up, onto a large serving platter, top with the white chocolate icing and then put the other cake on top.
To make the chocolate ganache icing, put the double cream in a saucepan and place over a medium heat and bring just to the boil. Remove from the heat and quickly add the butter, honey and the chopped chocolate. Leave to melt for a few minutes and then stir briefly to combine. Leave to cool slightly to allow it to become a little more viscous, and then spread over the top and sides of the cake. Smooth the edges using a palate knife and then decorate with the white chocolate curls.
If you want to store the cake overnight, place it in the fridge. Take the cake out of the fridge an hour before serving to bring it back to room temperature. If the cake is covered it will keep in the fridge for up to 2 days, but the sponge won't be as light and fresh. A cake like this is ideally eaten on the day it is baked. If you have any cake left over on day two, why not heat it up in the microwave and have 'hot' chocolate cake!