In a large bowl, whisk together the buttermilk, Dijon mustard, garlic powder, paprika, onion powder, cayenne pepper, salt, and pepper. Add the chicken thighs, ensuring they are well-coated. Cover and refrigerate for at least 1 hour (or overnight for best results).
In a separate bowl, combine the flour, cornstarch, baking powder, dried thyme, smoked paprika, and a pinch of salt. Mix well to ensure an even coating.
In a deep skillet or frying pan, heat about 1 inch of vegetable oil over medium-high heat until it reaches around 350°F (175°C).
Remove the marinated chicken from the fridge. Take each thigh out of the buttermilk mixture, allowing any excess to drip off before dredging them into the flour mixture, pressing down to form a solid coating.
Carefully place the coated chicken thighs into the hot oil, ensuring not to overcrowd the pan. Cook for about 6-7 minutes on each side, or until they are golden brown and the internal temperature reaches 165°F (75°C).
Once cooked, transfer the chicken to a paper towel-lined plate to drain any excess oil. Let it rest for 5 minutes before serving.
For the ultimate melt-in-your-mouth experience, serve the chicken with creamy mashed potatoes and your favorite green vegetables.