1 tablespoon baking powder
1 1/2 teaspoons kosher salt
12 tablespoons (1 1/2 sticks) cold unsalted butter, diced
1/2 cup cold buttermilk, shaken
1 cold extra-large egg
1 cup grated extra-sharp Cheddar
1 egg, beaten with 1 tablespoon water or milk
Maldon sea salt, optional
Preheat the oven to 425 degrees F.
Combine the buttermilk and egg in a small measuring cup and beat lightly with a fork. With the mixer still on low, quickly add the buttermilk mixture to the flour mixture and mix only until moistened. In a small bowl, mix the Cheddar with small handful of flour and, with the mixer still on low, add the cheese to the dough. Mix only until roughly combined.
Dump out onto a well-floured board and knead lightly about 6 times. Roll the dough out to a rectangle 10 by 5 inches. With a sharp, floured knife, cut the dough lengthwise in half and then across in quarters, making 8 rough rectangles. Transfer to a sheet pan lined with parchment paper. Brush the tops with the egg wash, sprinkle with salt, if using, and bake for 20 to 25 minutes, until the tops are browned and the biscuits are cooked through. Serve hot or warm.