Preheat your oven to 180℃ (350℉, Gas 4).
Take a large mixing bowl and use your fingertips to rub the butter into the flour, baking powder, and salt until the texture resembles rough breadcrumbs. Don't worry too much about achieving a perfectly even consistency, as small chunks of butter will help to give your cake a flaky texture.
Add the sugar, lemon peel or zest, and currants to the mixture and stir thoroughly.
Create a well in the center of the mixture and pour in the sour milk or kefir. Use a flat-bladed knife to mix the ingredients until they form a dough. Bring the dough together with your hands and knead briefly.
Place the dough on a baking tray lined with parchment paper, then use your hands to shape it into an oval about 2 cm thick. Alternatively, you can use a rolling pin, but using your hands will work just as well.
Use a knife to mark a crisscross pattern across the top of the cake, then bake it for approximately 30 minutes or until it has risen and become nicely browned on top.
If you wish, you can sprinkle a little caster sugar over the top of the cake.
Allow the cake to cool slightly before cutting it into pieces along the markings. This cake can be enjoyed either warm or cold, but it is best eaten within two days.