Preheat the oven to 425°F. Line a baking sheet with parchment paper or grease a cast-iron skillet.
Whisk together the flour, baking powder, sugar, and salt in a large bowl.
Grate the frozen butter with a box grater and add to the flour mixture, tossing to coat.
Make a well in the center of the flour mixture. Gradually add the buttermilk and mix just until a dough is formed.
Turn out the dough onto a lightly floured surface and pat into a square. Cut the dough in half and stack each half on top of each other. Repeat the process.
Pat the dough to ¾-inch thickness, and use a biscuit cutter to cut into 6 biscuits. Place the biscuits on the prepared baking sheet.
Whisk together the egg yolk and 2 teaspoons water in a small bowl. Brush the tops of the biscuits with the egg wash.
Bake in the preheated oven for 16 minutes or until the biscuits are golden brown, fluffy, and cooked through.