Put a baking sheet in the oven at 220c. 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 (I used a vanilla pod instead of extract) together in the microwave for 1 min or in the pan; it should be hot and may well go a little 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 4 cm thick, dip a 7 cm cutter into more flour, then stamp out four 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 minutes untill risen and golden. Best eaten just-warm or on the day.