Pat the chicken breasts dry with paper towels to remove excess moisture.
Season both sides of each chicken breast evenly with kosher salt, black pepper, and garlic powder.
Heat the olive oil in a 10-inch cast iron skillet over medium-high heat until shimmering but not smoking, about 2 minutes.
Add the chicken breasts to the skillet and sear without moving for 5-6 minutes until the underside is golden brown and releases easily from the pan.
Flip the chicken breasts over and add the butter, smashed garlic cloves, and thyme sprigs to the skillet.
Tilt the skillet slightly and spoon the melted butter over the chicken breasts continuously for 4-5 minutes, or until the internal temperature reaches 165°F (74°C).
Remove the chicken breasts from the skillet and let them rest on a plate for 5 minutes before slicing and serving.