DH reckons you can't smear a joint with roast beef butter unless it's the butter which has previously been smeared over the beef before roasting then when it's cooked it's drained from the joint for future use.
As MOnica says it’s basically a butter marinade. I have used tarragon and garlic butter, just massage in to the joint and refrigerate for 24 hours then cook. Best used for joints with little fat!!
Butter with beef extract or even dripping incorporated into it? Can't think how they could call it 'beef' butter unless it has some form of beef in it!
American here. I've lived in the North and South and both Coasts. Never heard of Roast Beef Butter or bread and butter roast. Some people will put a pat of butter on top of a good steak after grilling it to enhance the flavor but that's not called "beef butter".
I googled it and it simply means mixing butter with herbs and garlic, or whatever takes your fancy and spreading it over the join, leaving it to marinade for a few hours then roasting it and basting it with the butter.
I have been doing that to the Christmas turkey for years, mixing butter and salt and massaging it into the turkey breast so that the skin cooks beautifully crisp. DD and I love it.
The advert says, 'Sirloin of beef, matured for 30 days with a rich roast beef butter for extra succulence and intense flavour.'
So not a cut more suited to casseroling or pot roasting.
Maybe M&S have just made it up in the hopes there won't be an interfering old gran like me who would like them to tell us what on earth they're going on about.
I think it we would call it chuck steak. One of the other American names for meat from that front end of the beast is "English cut" so the term "butter beef" must be an American name.