Some candied yam recipes skip the simmering on the stove step and go right to baking—I prefer stove-top candying because the water/moisture can evaporate. It helps you have a gooey, soft end result that's never runny. Some kinds of butter have a higher liquid content than others, too. If your syrup seems too liquidy after thorough simmering, you can add a tiny bit of cornstarch to thicken it up.