1. Prepare Chicken: Sprinkle chicken thighs with salt and next 2 ingredients. Whisk together 1 cup buttermilk and 1 egg in a large bowl; add chicken, tossing to coat.
2. Pour oil to depth of 1 inch into a large cast-iron skillet; heat to 325°. Place flour in a shallow dish; dredge chicken in flour, shaking off excess. Fry chicken, in 2 batches, 5 to 6 minutes on each side or until golden brown and done. Drain on a wire rack over paper towels, and keep warm.
3. Prepare Biscuits: Combine bread flour and next 4 ingredients in a large bowl. Place cubed butter in a zip-top plastic freezer bag. Freeze flour mixture and butter separately 10 minutes or until well chilled. Whisk together 1 1/2 cups buttermilk and 1 egg in a small bowl.
4. Cut chilled butter into flour mixture with a pastry blender or fork until crumbly. Add buttermilk mixture, stirring just until dry ingredients are moistened.
5. Preheat oven to 450°. Turn dough out onto a lightly floured surface, and knead lightly 3 to 4 times. Pat or roll dough to 1-inch thickness; cut into 8 squares, and place on a parchment paper-lined baking sheet.
6. Bake at 450° for 15 to 16 minutes or until golden brown. Split biscuits; fill each with 1 cooked chicken thigh and desired toppings.