Season steak cubes all over with salt and pepper. Heat oil in a large skillet over medium-high heat. Add the steak and cook, stirring, until seared on all sides and pink in the middle, about 2 minutes. Remove from heat; add butter and garlic to the pan. Stir until the butter is melted and the garlic has softened, 1 minute more. Sprinkle with parsley, if desired.