I have a family brownie recipe that I have made zillions of times, but I finally added a note to the recipe that if I wait until it looks done and a toothpick comes out clean, it will be overdone, and just take it out of the oven when the times says. Because otherwise my tendency was to do much the same as you, and end up with slightly overdone brownies. And really, the failure mode of underdone brownies is much nicer.