Adjust oven rack to lower-middle position and preheat oven to 275°F. Spray a baking dish with nonstick spray; set aside.
Pound the thickest part of each breast with a meat mallet just until it has equal thickness on both ends.
4 boneless, skinless chicken breasts
Sprinkle ½ teaspoon kosher salt and ⅛ teaspoon black pepper evenly over each breast and place the breasts in the sprayed baking dish.
2 teaspoons kosher salt, ½ teaspoon freshly ground black pepper
Cover the dish with foil and bake until breasts register 145-150°F with an instant-read thermometer, about 20-30 minutes.
Turn off the heat and remove chicken from oven. Transfer to a plate and pat dry with paper towels.
In a large skillet, set over medium-high heat, heat the oil just until smoking.
1 tablespoon vegetable oil
Brush each breast lightly with melted butter and place them in the hot pan. Brush the top-side of each breast with butter.
2 tablespoons unsalted butter
Sear the breasts until the bottom side is golden brown, 3-4 minutes per side.
Reduce heat to medium-low, and use tongs to flip each breast over.
Cook until an instant-read thermometer registers 155-160°F, an additional 3-4 minutes.
Serve immediately or transfer breasts to a plate, cover with foil, and set the plate of breasts in the (turned off) oven to keep warm until ready to use.