Tip 350g self-raising flour into a large bowl with ¼ tsp salt and 1 tsp baking powder, then mix.
Add 85g butter cubes, then rub in with your fingers until the mix looks like fine crumbs then stir in 3 tbsp caster sugar. Leave in fridge while heating milk.
Put 175ml milk into a jug and heat in the microwave for about 30 secs until warm, but not hot. Add vanilla extract.
Make a well in the dry mix, then add the liquid and combine it quickly with a cutlery knife – it will seem pretty wet at first.
Scatter some flour onto the work surface and tip the dough out. Dredge the dough and your hands with a little more flour, then fold the dough over 2-3 times until it’s a little smoother. Pat into a round about 4cm deep.
Brush the tops with a beaten egg, then carefully place onto the hot baking tray. Set oven to gas 7 and bake for 10 mins