Preheat oven to 400ºF, with rack positioned in the middle.
Make a thick marinade. Mix the softened butter, miso, gochujang, garlic, red pepper flakes, vinegar and honey in a measuring cup or bowl, using a fork to blend everything together.
Line a large roasting pan with aluminum foil. Pat the chicken thighs dry with a paper towel or cotton cloth and place them skin-side down in the prepared roasting pan, then season them with salt.
Using your hands, rub half of the marinade on the flesh side of the chicken, then turn the thighs skin-side up and season with kosher salt. Then rub with the rest of the marinade.
Roast for 30 to 40 minutes, until the chicken is cooked through (an instant read thermometer should read at least 165ºF in the thickest part of the thigh).
If the chicken isn’t sufficiently brown, set your broiler to high and broil the chicken for 1 to 3 minutes, so that the skin and sauce brown (or char, as ours did—we did 3 minutes, so start with 1). Transfer chicken to a platter and squeeze a little lemon juice over it and serve.