Grate the frozen butter using a box grater, or cut it into cubes. Place grated butter back in the freezer while you prep the dry ingredients.
Whisk flour, baking powder, sugar, garlic powder, salt, and pepper together in a large bowl. Stir in the shredded cheese.
Add the frozen butter to the dry ingredients and combine using a pastry cutter, two forks, or just your hands, mixing until you get pea-sized crumbles. Place this into the fridge or freezer while you mix the wet ingredients.
Whisk together the milk and egg. Pour into the flour/butter mixture and mix until the dough starts to come together. Turn the dough out onto a floured surface, and knead with floured hands just a few times to get the crumbs to stick. The dough should be sticky, but if it's too sticky, add a tablespoon or two of flour.
Shape the dough into an even disc, about 8 inches around. Cut the dough into 8 even triangles using a sharp knife or bench scraper, and place them spread out on the lined baking sheet.
Put the baking sheet into the fridge for 15-20 minutes. Preheat the oven to 400°F while the scones are chilling.
Make the egg wash by mixing together the egg yolk and 1 tablespoon of milk. Before placing the scones in the oven, brush them lightly with the egg wash and sprinkle them with extra shredded cheese and flaky sea salt (optional). Bake for 22–25 minutes or until golden brown around the edges and lightly browned on top. Remove from the oven and let them cool for a few minutes on the baking sheets before enjoying.