Put a baking sheet in the oven at 220C/200C fan/gas 7.
Cut the butter in cubes
Put the flour, salt and baking powder into a food processor, then whizz in the butter until it disappears. Pulse in the sugar, tip into a large bowl, then make a well in the middle.
Warm the yogurt, milk and vanilla together in the microwave for 1 min or in a pan; it should be hot and may well go a bit lumpy-looking. Tip into the bowl and quickly work into the flour mix using a cutlery knife. As soon as it’s all in, stop.
Tip the dough onto a floured surface, then, with floured hands, fold the dough over a few times – just enough to create a smoothish dough.
Roll into a 7cm roll on more flour, then cut 4cm scones, flouring the cutter each time.
Beat the egg with 1 tbsp of milk
Brush tops with egg wash, scatter flour over the hot sheet, then lift the scones on. Bake for 12 mins until risen and golden. Best eaten just-warm, or on the day.