Preheat the oven to 425 degrees F.
Before you mix up your dry ingredients, take a spoonful of your flour and mix it with your grated cheese. This will prevent the cheese from sticking to itself and then it helps distribute it throughout the dough.
Start by combining the flour, baking powder and salt in a large bowl. Using a pastry blender or two knives cut your cold butter pieces into the dry mixture.You can alternately do this in your stand mixer fitted with the paddle attachment. Continue until the butter is cut into the size of peas. Do not overmix.
Combine the buttermilk and egg in a small measuring cup and beat lightly with a fork. With the mixer still on low, add the buttermilk mixture to the flour mixture and mix only until moistened. Add the cheese to the dough and mix only until roughly combined. Again, do not overmix.
If your dough is too wet, you can add a little more flour at this point, being careful to not add too much. You’re going to add more flour when you shape it so don’t be scared if the dough is a little on the soft side.
Turn the dough out onto a well floured work surface. Add additional flour at this point if needed to hold the dough together. Knead the dough lightly about six 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. These are actually really large biscuits, so if you want to you can make 2 cuts instead of that one lengthwise and get 12 biscuits.
Place the biscuits on a baking sheet lined with parchment paper. Brush the tops with egg wash, and bake for 20 to 25 minutes, until the tops are browned and the biscuits are cooked through.