Butter is butterfat, milk proteins, and water. Ground beef is lean cow meat and fat. When butter reaches its melting point (90-95°F), it clarifies, and turns almost exclusively into butterfat. And when that combines with cooked ground beef, and the two fats intertwine, the resulting reaction is not so much scientific as it is religious ecstasy. At least in Wisconsin.
The butter burger (essentially a regular burger with a scoop of butter melting over the meat) is a Wisconsin delicacy. In a land lousy with cow products -- cheese, milk, butter, beef -- the compulsion to combine them for certain glory is all but inevitable.
Wisconsin is one of the many places that lays claim to the invention of the burger in America, thanks to “Hamburger Charlie” Nagreen, who in 1885 allegedly decided that it’d be easier for people to eat the meatballs he was hawking at the Seymour Fair if they were between two pieces of bread. His sales song was catchy and involved many rhymes, but for our purposes, only the last two are important: “fried in butter, listen to it sputter.” Judging by that, it’s clear Chucky Burgers wasn’t just a talented songwriter; he might also be the originator of the butter burger.