There are instructions on the box for a thicker chewier brownie . Use one egg less and when baking be sure to test for doneness like this:
Insert toothpick in the corner about 1" from the pan - not 2" like the box says. When it comes out clean (no batter on the toothpick) it will be done. The center will still be a bit gooey, but will finish cooking as it is cooling. If you test in the center, the edges will be over cooked.