Preheat oven to 400℉. Mix all dry ingredients in a large bowl. Drop in butter cubes, and massage them into the flour until they are well broken up and the mixture looks crumbly or shaggy. Add in buttermilk and mix only until incorporated. Add your cheese last and mix it in until just combined. If you're making drop biscuits, you can go ahead and scoop/drop onto a parchment-lined pan now.
If you're cutting your biscuits, dump out dough onto a well-floured surface, and sprinkle a little extra flour on top to keep it from sticking to your hands. Knead the dough briefly and add small amounts of flour until it is less sticky and comes together. It should only take a minute; kneading too long will result in chewy or "squeaky" biscuits. But it's ok to work it until it's easier to roll out.
Add another sprinkle of flour to the counter and begin to roll out the dough. Roll until the dough is a uniform ½" thick.
Cut out your biscuits, making sure not to "twist" the cutter as you go through the dough. If you want double-stacked biscuits, fold the entire slab of dough in half onto itself, and then cut through both layers. Place your cut biscuits onto a parchment-lined pan. You can re-knead and re-roll the scraps until you've used it all up, just try not to overwork it as best you can. You'll probably end up with one ugly duckling biscuit at the end; Legacy Kitchen rules dictate that the cook gets to eat that one first as "test" when they're out of the oven and cool.
Mix the egg and water together until blended, and then brush it on the tops of the biscuits. Don't brush the sides, as this can impede the biscuits' rise. Bake on the middle rack for 12-15 mins. Then remove and transfer to a cooling rack.