You can make this casserole ahead of time up through step 4. Let it cool, cover it with plastic wrap and store in the fridge for up to 2 days. On the day you want to serve it, place it in a 350-degree F oven for 30 minutes to warm it through. Then, 10 minutes before you want to serve it, add the marshmallows on top. Slide it back into the oven for 5-10 minutes, until the marshmallows toast and turn lightly golden brown. As marshmallows sit, they dissolve fully into sugar. So, if you add and toast the marshmallows too soon, they will melt and dissappear! Warm the casserole through and add the marshmallows just 10 mintues before serving for best results with this recipe!