Preheat the oven to 180'C / Fan 160C / Gas Mark 4 and grease and line a 2lb loaf tin. Set aside.

Add the butter and sugar to a large mixing bowl and beat with an electric whisk until smooth. Then add the eggs and beat again until fully combined.

Sift in the flour, baking powder and xanthan gum and beat again until fully combined.

Take a second mixing bowl and pour half of the mixture in so you have half in each bowl. You can use scales if you want to be precise!

To one bowl, sift in the cocoa powder and add the milk. Mix until combined with no lumps. To the other bowl add the vanilla extract and beat until combined.

Spoon 2-3 tbsp of one of the cake mixtures in a line in the centre of the cake tin. Then spoon 2-3 tbsp of the second mixture on top of the first line, down the centre. Keep doing this, alternating the mixes, until you have used them all up - see the blog post above for photos to guide you.

Lightly nudge the tin so that the mixture reaches the edges (don't shake it too much or you'll ruin the marble effect) and then bake for 50-55 minutes until a skewer in the centre comes out clean. Cool completely on a cooling rack before serving.