Put a baking sheet in the oven at 220C/200C fan/gas 7. 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. Press out to about 4cm/1½in thick, dip a 7cm cutter into more flour, then stamp out 4 rounds, flouring the cutter each time. Squash the remainder lightly together, then repeat until the dough is used up. 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.