Method:
1 Beat butter in an electric mixer for 5 minutes or until very soft and pale.  Add chives, parsley, mustard, anchovy, capers, pepper, egg yolk, lemon juice and salt and pepper and mix until combined. Spoon butter onto a sheet of cling film and form into a sausage-shape using the cling film to roll and enclose, place in the fridge to set.
2 Heat a char-grill pan over medium-high heat, brush steak with oil and season with salt and pepper, cook steak for 4-5 minutes on each side for medium-rare, or until cooked to your liking. Rest for 5 minutes.
3 Transfer steak to a serving plate and slice a piece of the butter and place on top of steak to serve.