• Preheat the oven to 180C / Fan 160C / Gas Mark 4 and grease and line a 2lb loaf tin with a liner or baking paper. Set to one side.
• 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. Add the salt and beat once more until fully combined. Next, take a second large mixing bowl and pour half of the mixture in so you have half in each bowl.
• To one bowl, add the milk and cocoa powder and mix until combined with no lumps.
• To the other bowl add the orange zest, orange juice and orange extract and beat until smooth. Add a couple of drops of orange food colouring to this mix and beat until the mixture is a lovely orange colour.
• Using a spoon, place alternating dollops of the mixture into the tin. There doesn't have to be a logical pattern to it but I like to do mine in a sort of checkerboard to ensure they're fairly even. Do this until both mixtures are used up. Try not to leave any gaps as you don't want air bubbles.
• Next, take a skewer or dinner knife and run it through the mix, swirling the colours together. Don't over-mix or you'll love the marble effect - just a quick swirl will do nicely. I then like to lightly bang the tin on the worktop to make sure there are no trapped air bubbles.
• Place the cake tin in the oven and bake for 45-50 minutes until a skewer in the centre comes out clean. Cool completely on a cooling rack before icing.

To decorate:

• To make the icing, beat the ingredients in a bowl together until they form a smooth, thick paste. Spoon the icing onto the cooled cake and smooth out using the back of the spoon. A few drips look good! Sprinkle with some extra orange zest and serve.