My first suggestion would be to verify the temperature of your oven.
My second suggestion would be to make sure your bread pan is a decent
one, with no hot spots. Are you using a loaf pan or a brioche pan?
My third is that temps and times in recipes should always be looked
upon as estimates. There is no reason to leave the bread in any longer
than it needs to be in there. You know the joke about no one knowing
you're a dog on the Internet? Well, be careful with recipes you find
online, even from a trusted source like KA. Sometimes things just slip
through. Check out a few other recipes for brioche and compare them to
yours.
After you have tried the actions above, think about trying this:
After about 20 minutes, reach in and take the bread OUT of the pan you
are using. I am assuming from your recipe baking time, that the loaf
will be solid enough to perform this action. Check the internal temp
of the bread and then keep an eye on it. For a finished loaf, you are
looking for something between 190F and 195F for an enriched bread of a
size I am thinking you are making. Sometime I even take it a bit
higher, BUT, visual inspection is important, too.
And once you have the bread out of the pan, leave it out, so you can
more carefully monitor its color and temperature. Put it right onto
the rack.
Can you point me to the KA recipe you are using, please, so perhaps I
can help more? The recipe I see at first glance calls for 375F in the
oven and is linked below.
http://www.kingarthurflour.com/recipes/classic-brioche-recipe
Boron
PS....save me a taste! I LOVE brioche!