For the uninitiated, brioche is a sweet bread from France. It contains more egg and butter than other breads and the addition of sugar is why it tastes so sweet.
While it has been consumed for centuries, the use of it as a bun for burgers is relatively new and has been become something of a fad in the last few years much to the delight or horror of punters - as we can see from the viral Twitter thread.
But brioche is by far not the most unusual substitution for standard bread in a burger. Recently, restaurants have served patties between large pieces of lettuce, and others have even used ramen.
In the US, Burger King has started selling burgers using croissants as bread.