Combine the chicken, buttermilk, 2 teaspoons Old Bay and salt in a covered bowl and refrigerate for 2 to 6 hours.
Whisk the flour, cornmeal, baking powder, pepper and remaining Old Bay in a wide dish.
Let each thigh drip briefly, then pack the cornmeal mixture over every surface and along the edges.
Set the coated thighs on a rack for 15 minutes until the crust looks damp in spots instead of powdery.
Heat 1 1/2 inches peanut oil in a Dutch oven to 350°F.
Fry three thighs at a time for 12 to 14 minutes, turning every 3 minutes and holding the oil between 325°F and 340°F.
Remove each thigh when the crust is dark gold and the meat reaches at least 165°F near the bone, then drain on a rack for 8 minutes.