Sweet Potato Casserole with Marshmallows & Oat Streusel

The perfect holiday side dish, Sweet Potato Casserole with Marshmallows and Oat Streusel. So easy to make ahead this is a simple three part side dish is made up of mashed sweet potatoes, mini marshmallows, and a mouthwatering oat topping. The perfect addition to a Thanksgiving dinner!

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Make Ahead Sweet Potato Casserole with Marshmallows

An American tradition, sweet potatoes topped with marshmallows and plenty of sugar!

This dish actually originated in 1917 as a marketing ploy of a marshmallow manufacturer. The company Angelus Marshmallows hired the lovely people at the Boston Cooking School Magazine to use marshmallows in everyday cooking, as a result marshmallows ended up on sweet potatoes! (Also btw this is where marshmallows in hot chocolate started too). How it solidified to become a Thanksgiving tradition is anyone’s guess, but one thing I will say is that it has stuck!

My version of this beloved dish is made up of 3 layers:

1. Mashed Sweet Potatoes
2. An Oat Streusel
3. Marshmallows

Each is simple in it’s own form and these can also be made ahead of time to make whatever occasion all that much easier!

Supplies

For this dish you will need standard kitchen items such as knifes and measuring spoons but you will also need:

• A large pot for boiling the potatoes
• A potato masher OR a large spoon for mashing
• A baking dish for the final recipe, a 2.5 quart dish works best here
• If prepping ahead of time you will want containers for storing

Ingredients

Everything in this recipe is pretty standard:

• sweet potatoes – you will want medium sized potatoes, think about the average women’s hand, the potato should comfortably fit in the size of the hand
• kosher salt – kosher salt is best for cooking, sea salt would work okay for this recipe as well, in a pinch you can use table salt
• unsalted butter – use the unsalted variety, you will already be salting the potatoes
• milk – I use whole milk and it turns out great, although really any variety of milk should be okay here
• pumpkin pie spice – do yourself a solid and Make Your Own Homemade Pumpkin Spice (No Pumpkin Pie Spice? No fret, sub in half the amount of ground cinnamon)
• brown sugar – I use light in this recipe but there isn’t any reason you couldn’t use dark brown sugar if that is all that you have
• flour – all purpose flour, I have not tested this with any other variations so I cannot say for certain how a gluten-free variety would work
• rolled/old fashioned oats – Old Fashion Oats and Rolled Oats are the same thing, different brands just use the different wording, use this variety to give the full oat texture that is really nice
• baking powder – just a little for the streusel
• mini marshmallows – mini marshmallows work best for this recipe because they will melt quicker because they are smaller, if only regular marshmallows are available you can cut them into 4 pieces

For a complete detailed list of the ingredients continue towards the bottom of the page.

Instructions for How to Make Sweet Potato Casserole with Oat Topping and Marshmallows

This recipe is easiest when broken down into three sections. The sweet potato mixture, the oat topping/streusel, and the marshmallows. All three of these pieces can be prepped in advance!

Notes on Make Ahead

First off let’s just touch on some notes about how to prep this Sweet Potato Casserole. This dish can be prepped ahead of time up to 2 days in advance!

To prep ahead you will make and store the 3 different layers separately and then easily layer them when it is time to bake. I recommend storing the sweet potatoes in the dish that you will cook it in (it makes it so much easier). Then for the streusel a small container in the fridge. For the marshmallows you don’t technically have to ‘store’ them but if I am feeling extra organized I will measure them out and keep them in a separate bag for ease.

When it is time to make the dish, take the potatoes out of the fridge about 30 minutes before it is time to cook (especially if you are baking in the dish you stored it in). When it is time to put it in the oven quickly layer the marshmallows and then the streusel. And bake as stated below!

So let’s look at these layers:

For the Sweet Potatoes:

Peel and slice the potatoes into 1 inch rounds. Add the sliced potatoes and the salt to a large pot. Cover the potatoes with cold water. 

Transfer the pot to the stovetop over high heat. Cover and cook about 20-25 minutes until potatoes are tender and easily pierced with a fork. 

Carefully drain the water from the pot. Return the pot to the stove over medium heat uncovered for about 1 to 2 minutes to steam off any extra water in the pot or on the pototes to ensure they are dry. 

Remove from heat and add the butter, milk, and pumpkin pie spice to the pot. Gently mash the potatoes. Set aside until it is time to build the dish or transfer to an air-tight container to store in the fridge.

For the Oat Streusel:

Cut the cold butter into marble sized pieces. Add the butter, brown sugar, flour, oats, baking powder, pumpkin pie spice, and a pinch of salt to a medium sized mixing bowl. 

Using two forks, a pastry cutter, or clean fingers cut the butter into the mixture. Mixing the butter and dry ingredients together until crumbs form about the size of large peas. If making ahead transfer the streusel mixture to an air-tight container for the fridge. 

Ensure that the streusel stays chilled, if it gets too warm before it is time to cook the butter will start begin to melt and for best results you will want the butter to stay solid.

Making & Cooking the Casserole:

This casserole works best in a 2 ½ quart baking dish, although a close size will work as well.

The oven should be preheated to 350ºF. Note that this dish can be cooked simultaneously with others. Some other dishes might need to be at a different temp, so I tested it at 375ºF and it can be done it just took about 9-14 minutes instead, most importantly just watch the marshmallows on top to make sure they don’t get overdone.

Now to build the casserole, gently grease the baking dish. Transfer the mashed sweet potatoes to the dish. Spread the potatoes out evenly and smoothly. 

Layer the marshmallows over the sweet potatoes. Gently sprinkle the streusel overtop the marshmallows. 

Transfer to the oven and bake 18-20 minutes, being mindful to watch the top to ensure the marshmallows don’t get overcooked.

Remove from the oven and let sit at least 10 minutes. Letting it rest will give it time to solidify a bit, cutting in to early might cause the butter and marshmallows to be watery. 

Tips, Tricks, & Questions

Why is my casserole watery and how can I thicken it?

Your casserole may become watery for a couple different reasons:

• Not letting it rest and serving it too soon out of the oven. -The fix, just let it rest a little longer before continuing to serve.
• Not letting the potatoes dry off after being cooked thus being made into a watery sweet potato mixture resulting in a final watery base. (AKA don’t skip the step where you let the potatoes steam off the extra water after being drained) – The fix, you could try cooking it a little longer to see if it will dry out a little more, just keep an eye on the toppings to make sure it doesn’t dry out.

Can I Double it?

Yes! Just use a slightly bigger dish, if you are doubling it use at least a 9×13 baking dish.

Sweet Potatoes vs. Yams?

Are they the same thing? Different?

In what you see at the grocery store they are the SAME. The whole Sweet Potatoes/Yams is basically a marketing ploy back from the turn of the last century.

If you are at the grocery store, you are safe to grab either sweet potatoes or yams. It probably will depend on the store and location about what they call it.

In real life there is an actual difference between the two vegetables but real yams, close a yucca in texture and flavor are hard to find at stores in the United States and are often found in West African cooking.

Need more Thanksgiving and holiday dishes? You might like these:

• 5 Minute Cranberry Orange Relish
• Roasted Carrots with Dill
• Instant Pot Mashed Potatoes for a Crowd
• Roasted Pear & Pecan Salad
• Apple Cranberry Salad
• All Butter Pie Crust
• Sunflower Seed Pie

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