In a large bowl, rub the butter into the flour, baking powder and salt with your fingertips until you get the consistency of rough breadcrumbs. Don't be too particular about this, bits of butter will help to give a flaky texture to the cake.
Stir in the sugar, lemon zest or peel and currants.
Make a well in the centre and pour in the sour milk or kefir. Mix with a flat bladed knife until a dough forms. Bring the dough together with your hands and knead briefly.
Place on a lined baking tray and form into an oval with your hands and flatten to about 2 cm thick. You can use a rolling pin if you like, but hands work perfectly well for this bake.
Mark a criss cross pattern across the top with a knife and bake for about 30 minutes or until the cake is risen and nicely browned on top.
Sprinkle with a little caster sugar if liked.
Allow to cool a little, then cut into pieces along the markings. Delicious eaten warm or cold, but best consumed within two days.
Notes
Please note: calories and other nutritional information are per serving. They're approximate and will depend on serving size and exact ingredients used.