Preheat oven to 375°. Spray a 12-inch cast-iron skillet with cooking spray.
In a large bowl, whisk together flour, cornmeal, and salt. Using a pastry blender, cut in cold butter until mixture is crumbly. Add cold milk, and gently stir with a fork until well combined. Scoop dough by about 1 cupful, and drop into prepared pan, creating 5 biscuits.
In a small bowl, whisk together egg with 1 teaspoon water. Brush dough with egg wash.
Bake until golden brown, about 35 minutes. Serve warm.
Notes
*We used White Lily Unbleached Self-Rising Flour and Goya Fine Yellow Corn Meal.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/tomato-shrimp-gravy-cornmeal-cathead-biscuits/