Preheat the oven to 180°C and grease and line a standard loaf tin.
Add dairy free butter and caster sugar to a large bowl and beat with an electric whisk until creamy. Add the eggs one at a time and continue to whisk until combined. Add the vanilla and whisk again.
In a seperate bowl mix gluten free plain flour, baking powder, xanthan gum and salt. Add the dry ingredients to the wet ingredients and mix well. Add 2 tablespoons of dairy free milk and mix again.
Place a third of the mixture in a separate bowl and add cocoa powder and 1 tablespoon dairy free milk. Mix well.
Spoon half of the vanilla batter into the lined loaf tin followed by half the chocolate mixture. Use a skewer to swirl the mixtures together to create a marble effect. Spoon the rest of the vanilla mixture into the tin followed by the rest of the chocolate mixture. Swirl again.
Bake in the oven for 45-50 minutes, until a skewer comes out clean. Leave to cool in the tin before making your buttercream.