In a medium bowl, whisk together the buttermilk, cayenne, garlic powder, mushroom powder and kosher salt. Then add the chicken thighs to the buttermilk mixture and make sure all sides are coated with the marinade. Then cover the bowl with a lid or wrap and store in the refrigerator overnight to let the chicken marinate.
Fill a heavy bottom pot or dutch oven with frying oil and heat the oil to 350°F.
While the oil is heating, mix the flour and potato starch together in a shallow pan. Then pour about 1/4 cup of the buttermilk mixture into the flour starch mixture and mix with a spoon to create crumbles. Then dredge the marinated chicken thighs into the flour and make sure all sides and crevices are coated thoroughly. Then transfer the chicken to a wire rack. TIP: Shake off any excess buttermilk on the chicken before dredging. Then gently press the flour onto the chicken with your hands.
First-Fry: When the oil has reached 350°F, shake off any excess flour from the chicken and then gently drop the chicken into the pot. Fry the chicken for about 3 minutes. Then transfer the chicken to a wire rack. Repeat with the remaining chicken.TIP: Depending on how large your fryer is, it’s best not to overcrowd your frying pot with chicken because it will drastically reduce the temperature of the oil. I used a dutch oven and I fried two chicken thighs max at a time. Use a deep fryer thermometer in the dutch oven to keep an eye on the oil temperature. Second-Fry: Once all the chicken has been fried, increase the oil temperature to 375°F. Then fry the chicken again for about 2 minutes until they become golden brown and internal temperature reaches 165°F. Then transfer to a wire rack and let the chicken cool for 5 minutes.