Put the milk, yoghurt and vanilla paste into a jug and use a fork to mix them together.
Sieve the flour, baking powder and salt into a large mixing bowl, then add the cubed butter and work them together with your fingertips until the butter is completely mixed in. Add the sugar and stir it through, then make a well in the middle and pour in the milk. Use a knife to gradually mix the flour and liquid together, until it is just mixed.
Use your hands to bring the dough together into a ball. The trick is to work it as little as possible, but also get rid of any large cracks that will cause the scones to split as they rise. Flour your worktop and pat and shape the dough into a 4-centimetre thick square, which should be just big enough to cut out 4 scones. Dip the biscuit cutter into flour before cutting these out, then gently re-work the offcuts into a square again and cut out as many scones as you can. You should end up with 7 or 8. Space them out on a floured baking sheet.
Mix the egg yolk and milk together to make a rich egg wash. Brush it over the top of each scone, trying to avoid the sides. Bake for 14-16 minutes until they are well risen and firm if you press down on the top. Transfer to a wire rack to cool.
To make the jam: Hull the strawberries and chop them in half. Put them into a large saucepan with the lemon juice and water, and cook over a medium heat for 15 minutes, stirring often. Add the sugar and stir until it’s dissolved, then stop stirring and let it come to a rolling boil for 10-15 minutes, until it has thickened and reached 105C on a sugar thermometer, if you have one. Let it cool in the saucepan for 10 minutes before spooning it into an airtight jar - try not to stir it too much as it cools or you may create lots of air bubbles in the jam. You can keep it in the fridge for up to a month.