If the marshmallows dissolved, it could be that the cookie dough balls were not shaped correctly; there was too much cookie dough on top; the marshmallow was too big; or the cookies were over-baked. Be sure to use a 1 tbsp cookie scoop, and follow the step by step instructions on how to shape them above! The most important thing is to create a thick cookie dough base and thin cookie dough on top. This gives the marshmallow more support underneath, but allows it to peak through the top as it bakes!