Stir together first 4 ingredients. Cover and chill until ready to serve.
Combine 2 tablespoons lime juice and next 4 ingredients in a large, heavy-duty zip-top plastic bag. Add steak, and seal, turning to coat. Refrigerate 1 hour.
Remove steak from marinade, and discard marinade. Grill over high heat (400° to 500°) 7 minutes on each side (for medium rare) or to desired degree of doneness. Let stand 5 minutes before slicing. Top with chilled cilantro butter.