Preheat your oven to 170C (150C Fan). Line a 2lb loaf tin with greaseproof/parchment paper. With a loaf tin always make sure the parchment paper goes over the edges to help you lift the finished loaf out.
Cream the butter and sugars together until pale, light and fluffy. I use my electric hand mixer.
Add your eggs, vanilla, self raising flour, xanthan gum, baking powder and milk to the bowl and mix until fully combined. It shouldn't take more than about 90 seconds.
Split the cake mixture into two separate bowls, equally.
In a small bowl, mix your sieved cocoa powder and hot water together. Allow to cool briefly.
Once cooled add to one of your bowls of cake mix, mix it in thoroughly so the mixture becomes chocolatey.
At random spoon in your cake mixture into the tin. I tend to spoon one chocolate, followed by one vanilla etc this will ensure you get a marbled effect. Continue till the mixture is used up.
I like to then use a skewer to gently swirl my cake mixture a little more before placing in the oven.
Bake in the oven for 50-55 minutes, until cooked completely through.
Leave to cool in the tin for a while before placing on a cooling rack to completely cool.
Once cool make the icing. Melt your milk chocolate in the microwave. Once melted completely mix in your vegetable oil. Leave to cool slightly (5-10 minutes).
Then spoon the chocolate all over the top of the cake. Allow to set.
For a final (optional) touch melt white chocolate and using a piping bag drizzle it over the top. Allow all the icing to completely set. Enjoy!