Put the flour in a mixing bowl or a food processor. Add the butter and rub in with your fingertips or process until the mixture resembles fine breadcrumbs. Stir in the sugar and baking powder.
Stir the bicarbonate of soda into the yogurt then add to the flour mixture with the black treacle. Gradually mix in enough of the beaten egg to form a soft but not sticky dough. Knead lightly then roll out on a lightly floured surface until 2 cm (¾ inch) thick.
Working quickly, cut out 5.5 cm (2¼ inch) circles using a plain biscuit cutter. Transfer to a greased baking sheet. Reknead the trimmings and continue rolling and stamping out until all the mixture has been used. Add to the baking sheet and sprinkle the tops with a little extra flour or leave plain if preferred.
Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 6–8 minutes until well risen and browned. Transfer the scones to a napkin-lined basket and serve warm or cold, split and topped with crème frâiche and jam. They are best eaten on the day they are made.