Flaky Layered Buttermilk Biscuits

Heat the oven. Adjust an oven rack in order to the middle position and heat the oven to 220°C. Beat together the buttermilk and sour cream in a little bowl. Mix flour, baking powder, bicarbonate of soda, and salt and blitz till blended, around 2 seconds in the bowl of a food processor. Scatter cold butter uniformly over flour and blitz in short bursts till the mixture resembles coarse meal plus the largest butter pieces are around 5–6 mm at their widest, around 8 times. Move to a big bowl. Tip in the buttermilk mixture to the flour mixture and, with a spatula, fold till it has only just come together. Move the dough to a floured work surface and knead till it just comes together, adding extra flour as necessary. Roll the dough into a 12- by 20 cm rectangle with a rolling pin. Fold this right third of the dough over the center, next fold this left third over so you end up using a 12- by 10 cm rectangle using a bench scraper. Fold the top third down over the center, next fold the bottom third upwards so the whole thing is reduced in order to a 10 cm square. Push the square down and roll it out again in a 12- by 20 cm rectangle. Repeat the folding process once extra. (See notes for cheddar cheese plus scallion variation.) Roll the dough again in a 10- by 15 cm rectangle (around 2.5 cm thick). Cut eight 2 1.3 cm rounds out of the dough using a floured biscuit cutter. Move the rounds to a paper-lined baking tray, spacing them around 2.5 cm apart. Form the dough scraps in a ball plus work lightly 2 or 3 times, till there are no lumps left. Roll the dough out till it’s big enough to cut out 4 more 2 1.3 cm rounds, and move to the baking tray. Brush the tops of the biscuits using the melted butter plus cook till a deep gold and well risen, around 14 minutes, rotating the pan halfway through. Leave till it is cold for 5 minutes and serve.