Chicken: Heat salt, brown sugar, and 4 cups water in a large pot over medium, whisking, until salt and sugar dissolve, about 5 minutes. Remove from heat and stir in hot sauce and 8 cups ice water. Add chicken to brine, cover, and chill 4 hours. Combine 2 cups flour and 3 tablespoons Old Bay seasoning in a shallow baking dish and toss with your fingers to evenly distribute seasoning. Place buttermilk in a medium bowl. Remove chicken from brine and pat dry with paper towels.