Season both sides of the chicken breasts with salt, pepper, and garlic powder. Add skin side down to a pan with butter and cook without moving for 5 minutes.
Flip the chicken and add butter and crushed garlic. Let the butter melt and cook the garlic, then baste the chicken. Cook for 3 minutes, then remove and set aside on a plate.
Reduce the heat to medium-low and add the remaining butter and olive oil to the skillet. Add the sliced leeks and shallot and sauté until fragrant, about 3-4 minutes.
Place seared chicken breasts on top. Cover with a lid. Transfer the skillet to the preheated oven and bake for 25 minutes. Remove the lid and return the skillet to the oven. Cook for an additional 5-7 minutes