Mix together the flour, butter, xanthan gum and icing sugar in a large bowl and rub between your fingers to form breadcrumbs. Add the egg yolk and water and mix until it forms a pastry dough. Cover in cling film and chill for 20 minutes.
Preheat your oven to gas mark 6 or 200 degrees Celsius. Grease and line a 23cm tin with a removable base.
Roll out your pastry on a floured surface until it is larger than your tin. Carefully place this inside the tin and press lightly into the corner and sides. Bake blind (filled with baking beans) for 20 minutes, remove the beans, and bake for a further 5 minutes.
Heat up the cream and caster sugar in a saucepan and bring it to the boil. Remove from the heat and add the butter and dark chocolate. Continue stirring until the chocolate has melted.
Stir in the milk, this will give it the glossy shine, and pour it into the pastry case.
Leave to set for a few hours, then sprinkle with grated chocolate shavings to serve.