This is unusual brioche is halfway to being a Breton Kouign-amann. The difference is in the treatment of the dough, which is layered with sugar and butter. The basic dough is rolled out with butter in stages, like for croissants, but with 2 "turns" of sugar for one of butter (granulated and vanilla sugar).
The brioche is assembled in a round tin with spirals of dough that rise and caramelize during cooking. This recipe is rather rich, but what a treat!