Marinate the chicken thighs in buttermilk for at least 30 minutes.
In a bowl, mix flour, garlic powder, paprika, cayenne, salt, and pepper.
Heat oil in a skillet over medium-high heat.
Remove chicken from buttermilk, letting excess drip off.
Coat each piece of chicken in the flour mixture thoroughly.
Carefully place the chicken in the hot oil and fry for 5-7 minutes per side.
Once golden brown, remove and drain on paper towels.
Toast the brioche buns in the skillet until golden.
Spread mayonnaise on the bottom half of each bun.
Top with fried chicken and pickles, then add the bun top.