Mix the flour, salt, xanthan gum, baking powder and sugar together in a bowl. Rub in the butter with your fingertips until you have fine breadcrumbs. You can also do this by gradually pulsing the mixture in a food processor until it resembles breadcrumbs.
Whisk together the milk and whole egg and gradually mix into the flour mixture with your hands until you have a smooth dough. Mix in the sultanas, if using. Knead briefly to come together into a ball.
Gently roll out the scone dough until 2cm thick. Transfer to a baking tray lined with parchment and chill for 30 mins to firm up the dough – this makes them easier to cut out.
Remove the dough from the fridge and, using a 5cm cutter, cut out 6-8 scones (press the offcuts together and re-roll when you need to). Put the scones upside down (this will mean you get a neater top when baked) onto another baking tray lined with baking parchment, spread 2cm apart.
Whisk the egg yolk and evenly brush the tops of the scones, making sure that the egg wash doesn’t run down the sides of the scones otherwise they will rise unevenly. Put the scones on a tray and transfer to the freezer for 15 mins. Heat oven to 220C/200C fan/gas 7. Remove the scones from the freezer and brush the tops with the beaten egg again, then bake for 12-15 mins until golden brown. Eat just warm or cold, generously topped with jam and cream, if you like.