Heat the oven to 180C/160C fan/gas 4. Oil the base of two 18cm sandwich tins and line with baking parchment. Mix the flour, xanthan gum, cocoa, baking powder, bicarbonate of soda, a large pinch of salt and both sugars together in a large bowl.
Whisk the golden syrup, eggs, oil, yogurt, milk and coffee together in a jug. Gradually whisk the wet ingredients into the dry until you have a smooth mixture.
Pour the mixture into the prepared tins and bake for 30-35 mins until risen and firm to the touch. Remove from the oven and leave to cool for 10 mins in the tins before turning out onto a cooling rack.
While the cakes are cooling, make the icing. Beat the icing sugar, cocoa powder, butter and milk together for 5 mins until smooth and fluffy. Gradually pour in the melted chocolate while beating, scraping down the sides of the bowl to make sure everything’s included.
Spread half the icing over the middle of one sponge using a small palette knife. Sandwich the other sponge on top, with the flat bottom-side facing up. Spread the remaining icing around the top and sides of the cake, smoothing over with a palette knife. If you have any icing left, you can pipe some rosettes on top of the cake.