Use a knife to trim any membranes or fat off the chicken. Place the chicken thighs, one at a time, in a plastic freezer bag, removing as much air as possible. Pound the chicken to 1/4-inch thickness with a rolling pin or meat mallet. With a sharp knife, cut the pounded chicken In half. If the pieces are too large to fit a bun, you can trim off the ends. You can cook the smaller pieces to make chicken nuggets.
Put all the pounded chicken in the bag and add the buttermilk, Siracha, and 1 teaspoon of kosher salt. Refrigerate for 2 to 4 hours or overnight.
Pour the vegetable oil in a Dutch oven or high-sided pot. The oil should be about 1-inch deep. Heat the oil over medium high heat until shimmering, about 350 degrees F. Line a large plate with paper towels and set aside.
In a shallow bowl, whisk together the flour, cornstarch, baking powder, paprika, garlic powder, onion powder, remaining 1 teaspoon salt, and pepper.
Remove a piece of chicken from the marinade, letting the excess drip off and dip it into the flour mixture, turning a few times and pressing firmly so the flour sticks to the chicken. Set aside on a baking sheet and repeat with remaining chicken pieces.
Carefully place the breaded chicken into the hot oil, without crowding the pan. I recommend doing 2 pieces at a time. Cook turning a few times, until golden brown and crispy on both sides, about 4 to 5 minutes total.
Place the cooked chicken on the paper towel lined plate and continue frying the other pieces of chicken. Make sure the oil doesn’t get too hot or cool down, adjust temperature if necessary.
To make the sauce, in a small bowl, whisk together the mayonnaise, BBQ sauce, Dijon mustard, and Siracha, if using.
To make the slaw, place the cabbage in a medium bowl. In a small bowl, whisk together the vinegar and honey. Drizzle over the cabbage and season with salt and pepper. Use tongs to toss until well combined.
To toast the buns, spread butter on the inside of the buns. Place under the broiler for 2 minutes or until light golden brown. Don’t walk away, watch them closely.
Assemble the sandwiches: Spread some of the sauce on the inside of the toasted buns. Top the bottom bun with a piece of fried chicken. Top with slaw and a few pickles, if desired. Close the sandwich with the top bun and serve immediately.