Whisk flour, white sugar, baking powder, and salt in a mixing bowl. Cut in butter with a knife or pastry blender until the mixture resembles coarse crumbs.
Stir together 1/2 cup milk, sour cream, and currants in a small bowl; stir into flour mixture with a fork until just moistened and holding together.
Transfer dough to a lightly floured surface and pat into rectangle.
Fold the rectangle in thirds. Turn dough a half turn, gather any crumbs, flatten back into a rectangle, and repeat. Cut into eight pieces and place each scone on a silicone baking mat.
Whisk egg and 1 tablespoon milk together in a small bowl; brush mixture on top of each scone.
Bake scones in the preheated oven until risen and golden brown, about 15 minutes. Remove and transfer to cooling rack.
