Preheat the oven to 450°F/230°C. Rub both sides of the steak with 1/2 tablespoon of the oil, and sprinkle with the salt and pepper. Stir together the espresso granules and the ancho chili powder. Rub both sides of the steak with the espresso mixture, pressing to adhere. Let stand at room temperature about 30 minutes.

Place a large cast-iron skillet in the preheated oven until hot, about 10 minutes.

Remove the hot skillet from the oven; add the remaining 1 tablespoon oil, and place over high on the stovetop. Add the steak to the skillet, and cook, undisturbed, until a crust has formed, 2 1/2 to 3 minutes. Flip the steak and cook, undisturbed, for 2 1/2 to 3 minutes. Using tongs, hold the steak upright to brown the edges, and cook, turning occasionally, until the edges are browned, about 6 minutes.

Return the skillet to the preheated oven, and bake until a meat thermometer inserted in the thickest portion registers 145°F/63°C, 6 to 8 minutes for medium-rare, or to the desired degree of doneness. Remove the steak from the skillet; top with the butter, and let rest about 10 minutes before slicing.