Preheat the oven to 200°C with a baking tray in it.
Heat up the milk so that it is just warm. Add the squeeze of lemon to the milk and the vanilla.
In a bowl, or in the bowl of a stand mixer, add all of the dry ingredients (flour, baking powder, salt, sugar, lemon rind) and the butter. If using a stand mixer, use the paddle attachment and mix on low until the mixture resembles breadcrumbs. If using a bowl, rub the butter into the flour with your fingertips until it resembles breadcrumbs.
Pour in the milk mixture and sourdough discard and incorporate to form a shaggy dough. It is important not to overmix the dough as it will become tough. The dough will be quite wet, this is normal.
Tip the dough out onto a well-floured, clean worksurface. Make sure the dough is well coated with flour so that it does not stick to your hands. Form the dough into a thick circle (around 2.5cm thick) and pat so that the surface is even.
Using a bench scraper or large knife, cut the dough into eight equal triangular parts.
Beat one egg and add a little water to make and egg wash.
Remove the hot baking tray from the oven and place the uncooked scones evenly on the tray. They should not be too close together as they will rise.
Brush the scones with the egg wash, using a pastry brush.
Bake in the oven for 18-20 mins until golden brown.
Leave to cool completely before glazing.