For the Icing
15g cocoa powder, sifted
1-2 tbsp milk
100g icing sugar, sifted
25g white chocolate, melted
Prepare a 900g loaf tin by greasing and lining with baking parchment. I take the easy option and use preformed loaf tin cake liners.
Weigh the butter, sugar, flour, baking powder, eggs, milk and vanilla extract into a large bowl.
Mix with an electric mixer for about two minutes until the mixture is well creamed and blended.
Divide the mixture into two using a second bowl and set aside.
Using a small bowl or ramekin dish blend the cocoa powder and hot water together until smooth. Leave to cool for a minute or two.
Add the cocoa mixture to half of the cake mixture and mix until well incorporated.
Spoon random amounts from each bowl into the loaf tin, give a shake to combine, marble the mixture with a skewer if you want, and level the top.
Bake for at least 50 minutes until the case has risen and is springy to the touch. Cool in the tin for a few minutes, then turn onto a wire rack, remove the parchment or cake tin liner, and leave to continue cooling.
For the icing, melt the butter in a small pan with the cocoa powder, stirring and cooking gently for about 1 minute.
Mix in the milk and icing sugar and take off the heat.
Leave to cool and thicken. You can prepare the icing whilst the cake is still cooking so that it gets a chance to thicken.
When the cake is cold cover the top with the chocolate icing, then drizzle with the melted white chocolate. I used a spoon, but if you're feeling a little less lazy you could use a piping bag.