The middle of your pan will be more gooey than the edges - just like any brownie recipe. If you are particularly concerned about your brookies being underdone, you can leave them in the oven a little longer, but do keep in mind that these will firm up a bit as they cool as well. If you try to slice these straight from the oven, you will find that they seem almost molten, but if you leave them to set at room temperature for an hour or so, they'll improve.