A hound on the SF board posted a link to a video about Maverick and its "butter burgers." http://chowhound.chow.com/topics/779721 Basically, they grind beef with cubes of butter, cook them sous vide, then sear them in a cast-iron pan.
This got me thinking - although I think 7-bone chuck is about ideal for burgers, there are plenty of other inexpensive cuts that taste great but are too lean for a proper hamburger. I've always passed them up for grinding because of the fat content, but this seems to open up a whole new world of possibilities.
I'm not sure I'd go with Maverick's recipe. The chuck he's using looks like it's fatty enough already, and the sous vide step serves no obvious purpose. But taking a bottom sirloin or a flatiron roast or some other tasty cut and upping the fat content with a stick or two of butter? Sounds like a winner!
Has anybody tried adding butter to beef as it's being ground? What kind of results did you get?