Oh no! This can happen, especially when going for super fudgy brownies. It may be that you've not left your fluffernutter brownies long enough to cool - either try waiting a little longer or you can place the pan in the fridge for them to cool more quickly. It may also happen if you've not baked the brownies long enough. The toothpick should be mostly clean when inserted in the centre - that means flecks of brownie on it. It should not still be a liquid batter in the centre - if it is leave the brownies to cook a bit longer.