*The casserole can be made in advance and either baked just before serving OR baked in advance and then rewarmed before serving. Add the marshmallow meringue just before serving. *I have found that if you let the marshmallow sit longer than 45 minutes the maple starts to separate from the whipped whites. it's really important to pipe the marshmallow onto the casserole almost immediately after you make it. Once the marshmallow is toasted, it will not change shape or form. *If you would prefer to make a sweeter more traditional marshmallow, use 1 cup granulated sugar in place of the maple syrup.