Place the softened butter and sugar in a mixing bowl. Beat them together until the mixture looks pale and fluffy. This takes about 2–3 minutes with a wooden spoon or hand mixer.
Separate your egg and set the white aside for later. Mix the yolk into the butter mixture along with the lemon zest. Stir until it’s fully combined.
Add the flour to the bowl and gently mix until you get a crumbly dough. Sprinkle in the currants. Pour in 1 tablespoon of milk and press everything together with your hands. If it’s too dry, add the second tablespoon of milk.
Wrap the dough in cling film and pop it in the fridge for 20–30 minutes. This makes it easier to roll and helps the biscuits hold their shape.
Preheat your oven to 180°C (160°C fan) / 350°F. Lightly flour your surface and roll out the dough to about ½ cm thickness. Use a round cutter to cut out the biscuits and place them on a lined baking tray.
Beat the leftover egg white just a little and brush it over the top of each biscuit. Sprinkle a pinch of sugar on top for a sweet crunch after baking.
Bake the biscuits for 12–15 minutes until they’re a light golden color. Keep an eye on them—they cook quickly.
Let them rest on the tray for a few minutes, then move them to a wire rack to cool completely.