Add onion, garlic, hot sauce, garlic salt, smoked paprika, and cajun seasoning to a food processor. Blend until a uniform puree/paste forms.
Add chicken to a large bowl and coat in the puree, kosher salt, and black pepper, to taste. Pour the buttermilk and pickle juice over the chicken and press it down to ensure it stays fully submerged. Cover with plastic wrap or a lid and allow chicken to soak in buttermilk brine for 2 hours (up to overnight) in the refrigerator.
Add flour, cornstarch, baking powder, garlic salt, garlic powder, smoked paprika, onion powder, oregano, thyme, and Sazon to a large bowl. Whisk together. Remove chicken from the buttermilk, removing as much of the marinade as possible. Then pat dry with paper towels – save the buttermilk mixture.
Add eggs to the seasoned buttermilk mixture and whisk together until combined. Place chicken into the flour mixture, then dunk into the buttermilk marinade, then dip back into the flour mixture. Make sure to really coat the chicken in the flour coating, the double dredge is what gives the chicken its crispy crust. Tap off any excess flour.
Place onto a wire rack and let the chicken rest at room temperature while the oil heats. Heat oil over medium heat in a deep skillet or dutch oven. Oil should be about 4-5 inches deep in the pan. Heat to roughly 350-375 degrees Fahrenheit. Fry 2-3 pieces of chicken at a time and let the chicken cook until crispy and deep golden brown, about 8-10 minutes. (Cooking time will vary with the thickness of the meat. To ensure chicken is thoroughly cooked, use a meat thermometer to ensure the internal temperature of the chicken is 165 degrees F.)
Remove from the oil and place onto a clean wire rack. Let the excess oil drip off of the cooked chicken.
Melt butter in a skillet, then add ¼ cup of the frying oil. Add the garlic powder, smoked paprika, chili powder, brown sugar, honey, and kosher salt. Cook for 2-3 minutes until a spicy paste forms, then add in chicken broth and stir until fully incorporated.
Brush the spicy sauce on top of the chicken.
Combine all of the coleslaw ingredients in a bowl and mix. Then mix together the spicy mayo ingredients in a separate bowl.
Slather the mayo on top of both pieces of bread. Place the sliced pickles and breaded chicken on top, then add a generous dollop of coleslaw. Serve with your favorite sides!
Notes
Don't skip the marinate!! Not only does this build flavor, but it also tenderizes the meat. If you love juicy boneless chicken thighs, be sure to follow this step in the cooking process.
Use a thermometer when frying! This can make or break the recipe. Too hot, the chicken will brown too quickly and likely be raw on the inside. Too cool and the chicken will be greasy. An instant read thermometer or candy thermometer ensures the oil temperature is just right for that nice crispy chicken.
Don't overcrowd the pot! Frying too many pieces at once drops the oil temperature and leads to greasy chicken that won't cook evenly. Fry in batches and keep finished pieces warm in a low oven (200°F) on a wire rack.
After dredging, place the chicken on a wire rack and let it sit for 15–20 minutes before frying. This helps the flour adhere better and prevents it from sloughing off in the oil.