If the chicken breasts are thick, cut them in half to create thinner filets. Pat dry with a paper towel to remove excess moisture.
In a large bowl, whisk together the flour, cornstarch, garlic powder, chili powder, salt, mustard powder, black pepper, and baking powder. Set aside.
In another deep bowl, whisk together the buttermilk, egg, hot sauce, pickle juice, and baking powder.
Add 3 to 4 tablespoons of the wet batter into the dry mixture. Mix well to create a clumpy, flaky texture—this helps make the breading extra crispy!
Dip each chicken filet into the dry mix, then into the wet batter, then back into the dry. Use one hand for the wet mix and the other for the dry to keep the breading process clean. Let the breaded chicken rest for 10 to 15 minutes before frying.
Heat oil to 350°F in a frying pan, about 1.5 to 2 inches high. Carefully place the chicken into the hot oil. Do not overcrowd the pan. Fry for about 7–8 minutes, flipping halfway, until golden brown and crispy. Transfer to a paper towel-lined plate to drain excess oil.
Reserve ¾ cup of the frying oil in a heatproof bowl. Stir in brown sugar, chili powder, cayenne, mustard powder, garlic powder, and salt. Mix well until smooth.
Dunk each fried filet into the spicy oil, making sure it’s fully coated. Place the spicy chicken on a toasted bun, drizzle with Dave’s Hot Chicken Sauce, and top with pickles. Enjoy the ultimate homemade hot chicken sandwich!