Preheat the oven to 375 degrees F (190 degrees C). Line a baking sheet with parchment paper.

Meanwhile, soak raisins in hot water for 10 minutes; drain and set aside.

Whisk flour, sugar, baking powder, and salt together in a mixing bowl. Cut in cold butter with 2 knives or pastry blender until mixture resembles coarse crumbs. Stir in milk until moistened; add raisins. Turn dough out onto a lightly floured surface. Pat or roll dough out into a 3/4- to 1-inch thick round. Cut into 8 biscuits with a biscuit cutter and place onto the prepared baking sheet. Beat egg with 1 tablespoon water in a small bowl to make an egg wash; brush on top of each biscuit.

Bake in the preheated oven until biscuits are golden brown on top, about 15 minutes.