Heat butter in a 12-inch heavy skillet over high heat until foam subsides. Meanwhile, pat ham steak dry and sprinkle all over with 3/4 teaspoon pepper. Sauté, turning once, until browned in spots and heated through, 4 to 5 minutes total. Transfer ham to a platter.
Add bourbon to skillet and carefully ignite with a kitchen match. When flames subside, add cream, mustard, and coffee powder (if using), scraping up any brown bits. Simmer until slightly thickened, 1 to 2 minutes. Pour some of sauce over ham and serve remainder on the side.