Butter a 20cm square baking tin and line with baking parchment, leaving an overhang. Rub the butter into the flour using your fingertips. Stir in a pinch of salt and the icing sugar, then bring together using your hands and 2-3 tbsp cold water. Cover and chill for 30 mins.

Mix the currants with the 70g caster sugar and 1 tsp water. Stir in the apple, if using. Heat the oven to 180C/160C fan/gas 4. Cut the chilled pastry in half and roll out one half until it's large enough to line the base of the tin. Spoon the fruit mixture over the pastry, leaving a border around the edges. Brush the exposed border with a little milk. Roll the rest of the pastry out until it's just larger than the base, then lay it on top and press around the edges to seal. Trim any excess. Brush over more milk, then scatter over the 1 tbsp caster sugar. Bake for 35-40 mins. Leave to cool in the tin, then cut into 16 squares. Will keep in an airtight container for three days.