Cut off the top or “head” of the brioche using a bread knife.
Carefully cut an opening by pushing the knife vertically through the top, ½ in. (1 cm) from the crust, to within ¾ in. (2 cm) of the base. Take care not to cut all the way through the base.
Cut around the inside of the brioche, leaving a ½-in. (1-cm) border.