Preheat oven to 180ºC (350ºF). Line a large baking tray with baking paper and set aside.
In a high-speed food processor, add flour, sugar and baking powder. Pulse a couple of times to blend together.
Add chilled cubed butter. Pulse for 15 seconds until mixture resembles coarse crumb.
Through the vertical shoot, drizzle a tablespoon at a time of the milk then pulse the mixture. Once all six tablespoons of milk have been added, keep pulsing mixture until it begins to stick together into a rough dough, not a solid ball of dough (refer to images above). If the dough is still too dry, add a little more milk until it comes/sticks together.
Turn dough onto a lightly floured bench top or large board. Bring together to a ball of dough, you may need to knead it a little. Then flatten it out into a rough thick rectangle shape.
Sprinkle flour under the dough again and on the rolling pin, enough flour as the dough can stick to the surface.
Roll dough out to a large rectangle, 4mm in thickness and measuring 40cm in length and 30cm in width (15 inch x 11 inch).
Sprinkle currants over one half of dough, I used the top half (refer to above images for step by step). Spread currants out to the edges.
Fold other half of dough up and over the currants, matching up the edges evenly.
Flour under dough and rolling pin. Roll dough out to a large rectangle again, measuring about 35cm in length and about 25cm in width (13 inch x 9 inch) and about 4mm in thickness.
Trim edges using a long sharp knife. The finished rectangle should measure about 30cm in length and 20cm in width (11 inch x 7 inch).
Take a fork and lightly prick the dough all over, the fork doesn’t have to go completely through the dough, just prick the surface.
With that same knife, evenly slice the dough down the middle, lengthways, to create two rectangle pieces.
Slice into long biscuits measuring about 10cm in length and 4cm in width (4 inch x 1.5 inch).
Carefully pick up and transfer biscuits to lined baking tray. Leaving a small gab between each.
In a small mixing bowl whisk together egg and milk.
Using a pastry brush, brush each biscuit with a light layer of egg wash.
Sprinkle each with some caster sugar.
Bake for 10 minutes then rotate tray and bake for a further 10-12 minutes until light-medium golden all over.
Remove from oven and allow to cool on tray for 5 minutes.
Carefully transfer biscuits to a wire rack to cool completely.
Serve and enjoy!