If you are a single diner like me, half the steak will serve for dinner, the other half can be used the next day for a great Philly Cheese steak (slice and reheat with shredded Swiss cheese and serve on a roll with fried onions and peppers).
Mix soy sauce, brown sugar, pepper, and olive oil. Pour into bag with steak, making sure to coat both sides. Seal bag (get as much air out of the bag as possible).
Let bag sit at room temperature for 30 minutes, turning several times.
Heat a large heavy skillet, add butter when it is very hot. Add steak to skillet and cook over high heat until charred on one side (about 6 minutes). Turn and cook until charred on the other side (about 3 to 5 minutes more).
The result will be charred on the outside, slightly pink in the middle. Serve immediately on a well heated platter.