Hi, Kathleen! This kind of mushrooming does happen sometimes with this type of bread. You may have luck removing the loaf from the pan a little early (when it's around 190° or so), placing it on a baking sheet, and popping it back in the oven for five minutes to firm up the walls. Baking your brioche in a parchment sling inside your loaf pan will make this process of removing and transferring the bread much easier and less prone to burn-inducing slips and mishaps. Happy baking! 

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