Combine currants and liqueur:
In a small bowl, mix the currants and liqueur, then set aside to soak.

Make the dough:
In a medium bowl, whisk together the flour, half of the sugar, and salt. Add the grated cold butter and cut it into the flour mixture using a pastry cutter, knife, or your fingertips (be careful not to melt the butter). Use a fork to incorporate the milk, then bring the dough together with your hands. Flatten into a rectangle, wrap in baking paper or plastic wrap, and chill for 30 minutes.

Preheat the oven:
Preheat the oven to 180°C (160°C fan-forced) / 350°F. Line a baking tray with baking paper.

Prepare currants:
Drain the currants well, discarding the liquid. Set them aside in a strainer to drain further.

Prepare the cinnamon sugar:
In a small bowl, combine the remaining sugar and ground cinnamon.

Roll out the dough:
Cut the dough in half. On a lightly floured surface, roll one half to about a 20 cm square (8×8 inches), using the base of an 8-inch tin as a guide. Carefully transfer this dough to the prepared baking tray.

Assemble the biscuits:
Scatter the drained currants over the rolled dough, then sprinkle half of the cinnamon sugar mixture on top. Roll out the second half of the dough and lay it over the top. Gently press down all over with your hands, then trim off the rough edges.

Add the egg wash:
Brush the top layer of dough with the beaten egg yolk mixture, then sprinkle the remaining cinnamon sugar on top.

Bake:
Bake for 25-30 minutes until the top is golden and crisp.

Cool and slice:
Use the ends of the baking paper to transfer the biscuit to a cutting board. Cut into rectangles and allow to cool completely.