Combine chicken, buttermilk, 2 teaspoons Old Bay, and salt in a covered bowl. Refrigerate 2 to 6 hours.
Whisk flour, cornmeal, baking powder, pepper, and remaining Old Bay in a wide dish.
Let thighs drip briefly; pack cornmeal mixture over every surface and edge.
Rest on a rack 15 minutes until coating looks damp in spots, not powdery.
Heat 1 1/2 inches peanut oil in a Dutch oven to 350°F.
Fry three thighs at a time, 12 to 14 minutes, turning every 3 minutes and maintaining oil at 325°F to 340°F.
Remove thighs when crust is dark gold and meat reaches at least 165°F near bone. Drain on a rack 8 minutes.