Preheat oven to 180°C(160°C fan). Sieve flour, xanthan gum, baking powder and cocoa powder into large mixing bowl with sugar, salt & orange zest.

In another bowl combine melted (& cooled) butter, milk, eggs and orange juice, then pour into dry mixture & mix till smooth. Add in chocolate chunks & stir.

Pour into buttered baking dish and smooth with spatula. Measure 300ml boiling water in jug, then add in muscovado sugar & cocoa powder & combine. Pour liquid over cake mix in dish. Cook for 30 mins and serve warm!