These bars will not get firm in the oven, which can make it difficult to tell when they are finished baking. To check for doneness, gently shake the pan and watch how the surface of the bars moves. If the surface jiggles as a whole like jello, then they are likely done. If the surface looks liquid-like, then the bars need to bake more. You can also try inserting a knife in the center of the pan and if it comes out clean they are done. These bars will get firm while they cool, which is why it is so important to let them cool for at least 2 hours before you slice them.