Mince 3 garlic cloves. Finely grate 1 ounce Parmesan cheese on the small holes of a box grater (about ¼ cup) if needed, or measure ⅓ cup store-bought grated. Pick the leaves from 3 fresh parsley leaves and finely chop until you have 1 teaspoon.

Cut 1 ½ pounds boneless chicken into 1-inch pieces. Place in a large bowl, add ¾ teaspoon kosher salt and ¼ teaspoon black pepper, and toss to coat.

Melt 2 tablespoons of the unsalted butter in a large cast iron or stainless steel skillet over medium heat. Add half of the chicken in a single layer. Cook until golden-brown and the chicken easily releases from the bottom of the pan, about 4 minutes. Toss and cook until golden-brown on the second side and cooked through, about 2 minutes more.

Transfer the chicken to a plate. Melt 1 tablespoon of the unsalted butter in the skillet and repeat cooking the remaining chicken. Transfer to the same plate.

Reduce the heat to low. Add the remaining 1 tablespoon unsalted butter, garlic, chicken, and any accumulated juices on the plate to the skillet. Cook until the butter melts and the garlic is fragrant, about 1 minute. Remove the skillet from the heat, add the Parmesan cheese, and toss to coat. Garnish with the parsley, more Parmesan cheese, and red pepper flakes if desired.