Preheat the oven to 425°F (220°C). Lightly butter a 10-inch cast-iron skillet.

In a large mixing bowl, whisk together the self-rising flour, salt, and sugar.

Add the cold butter and use a pastry cutter, two forks, or your fingertips to work it into the flour until the mixture resembles coarse crumbs with a few pea-sized pieces of butter remaining.

Stir in the shredded cheddar cheese and chopped fresh sage until evenly combined.

Pour in the buttermilk and gently stir with a rubber spatula or wooden spoon just until no large pockets of dry flour remain.

Turn the dough onto a lightly floured work surface. Gently press it together into a rough rectangle about 1 inch thick. Fold the dough in half, rotate it 90 degrees, and repeat this process 3 to 4 times.

Using a 2¾-inch biscuit cutter dipped in flour, press straight down without twisting to cut out 6-7 biscuits.

Place the biscuits in the prepared skillet so they are just barely touching.

Chill (Optional, but Recommended) for 10–15 minutes before baking.

In a small bowl, whisk together 1 egg with 1 teaspoon of water or buttermilk. Lightly brush the tops of the biscuits with the egg wash.

Bake for 18–22 minutes at 425℉, or until the biscuits are tall, golden brown on top, and baked through.

Remove the skillet from the oven and, if desired, brush the tops with melted butter while the biscuits are still warm. Serve immediately.