Take steaks out of the refrigerator 20 minutes before cooking to allow them to come to room temperature.
Preheat oven to 425ºF; place a large cast iron skillet in the oven as it preheats. In a small bowl, combine chipotle, cocoa, garlic powder, cumin, smoked paprika, salt and pepper. Rub the mixture all over the steaks.
Carefully remove skillet from oven and place over medium-high heat. Add oil to skillet and swirl over the surface. Place steaks in hot skillet and sear for 2 minutes. Turn and sear 2 minutes longer.
Turn again, place a piece of butter on top of each and return skillet to oven. Roast until an instant read thermometer stuck into thickest part of steak reads 135ºF (for medium rare), 6 to 10 minutes, depending on thickness.
Transfer steak to a cutting board, tent with foil and let rest for at least 10 minutes before slicing and serving. Pass sour cream on the side, if desired.