Preheat your oven to 170°C Fan/190°C/375°F/Gas mark 5.
Take your large baking sheet and cover the base with a sheet of greaseproof (parchment) paper (or grease with some extra butter and then sprinkle over some extra flour).
Sift your flour, baking powder and salt into your large mixing bowl.
Dot over your chopped cold butter and rub it into the flour with your fingertips holding it up high and letting it fall as you do so, to keep it full of air. Stop once the mixture becomes sand-like. To double check that there are no large lumps of butter left, give the bowl a shake to bring any large pieces to the surface.
Add your egg yolk and rub that in too, in the same way.
Mix your buttermilk and milk together.
Make a well (dent) in the centre of your flour mixture and pour in half of your buttermilk/milk mixture. Mix and combine using a fork, until some large lumps are formed but there is still plenty of dry mixture left.
Mix in your demerara sugar, until evenly distributed.
Make a well in the middle again and add the rest of your buttermilk and milk mixture and your currants.
Mix with your fork again and keep mixing until all of the dry mixture has been combined, then go in with your hands and pull it all together into a ball. If you find the mixture is not easily coming together and is a bit dry, add another 1-2 teaspoons of milk or buttermilk, the mixture should not be sticky but should be pliable and bind together. Do not knead it though as this will affect the texture of your Paris Buns.
Place the ball onto your work surface and flatten it a little so that the surface area is level. Cut it down the centre and then each side into 3 equal pieces, then roll each piece into a rough ball. Push any protruding currants into the buns, to stop them from burning.
Place balls onto your lined baking tray, flattening them slightly and ensuring a gap of an inch and a half apart.