How to Make It
In a large resealable plastic bag, combine the pickle brine and the chicken thighs. Seal the bag and refrigerate the chicken for at least 8 hours or overnight.
Meanwhile, in a small bowl, whisk the schmaltz with the gochugaru, sugar, garlic powder, 1 teaspoon of the cayenne and 1 teaspoon of salt.
Remove the chicken thighs from the marinade and pat dry. In a shallow bowl, whisk the flour with the white pepper, paprika, 1 tablespoon of salt and the remaining 1 teaspoon of cayenne.
Set a rack over a baking sheet. In a large, deep skillet, heat 1/3 inch of oil until shimmering. Dredge half of the chicken in the seasoned flour, shaking off any excess. Fry the chicken thighs skin side down over moderate heat until well-browned and crisp, about 4 minutes. Turn the chicken and cook until no trace of pink remains, about 4 minutes. Transfer to the rack. Repeat with the remaining chicken.
Generously brush the fried chicken thighs all over with the gochugaru paste. Put 4 slices of bread on a work surface and arrange 2 chicken thighs on each. Top the chicken with sliced pickles, close the sandwiches and serve.
Schmaltz is available in the refrigerated section of most supermarkets.