Preheat oven to 425 degrees F. Line a large rimmed baking sheet with parchment paper, a silicone baking mat, or foil.

In a small bowl, use a fork to smash together the white miso paste, brown sugar, rice vinegar, softened butter, red pepper flakes, ground ginger, and black pepper until smooth and well combined.

Pat the chicken thighs completely dry with paper towels. Trim any excess fat with kitchen scissors. Arrange the thighs on the prepared baking sheet.

Spoon a dollop of the miso glaze onto each thigh and use your hands to spread it evenly over the top, ensuring thorough coverage across each piece.

Bake for 25 minutes until the miso glaze is caramelized and deeply golden-brown. If butter pools on the pan, spoon it back over the chicken once or twice during baking.

After removing from the oven, add a small splash of water to any caramelized bits on the pan, scrape them up, and brush the concentrated drippings back onto the chicken.

Serve hot topped with sliced green onions alongside steamed rice, noodles, or your favorite vegetable sides.