In this specific zucchini brownies recipe the key is not to over-bake them. You need to remove them from the oven when the top is just set, even though the batter may still seem a little jiggly overall. If you keep baking them at this point the brownies will be more dry and fluffy cake-like. Whereas slightly under-baked (not raw) brownies will be nice an fudgy and chewy, especially if you let them cool fully first at room temperature and then chill in the fridge before slicing.