Coat sweet potatoes in olive oil and pierce skins numerous times with a fork. Place sweet potatoes on a baking sheet lined with parchment paper.
4 large sweet potatoes, 1 tablespoon olive oil
Bake sweet potatoes for 1 hour and 10 minutes or until soft and skins are slightly wrinkled. Allow sweet potatoes to cool for 10 minutes.
Cut sweet potatoes in half. Using a spoon or a cookie scoop, remove the inside of the sweet potatoes, being careful to leave a 1/4 to 1/2 inch ring around the sides so that the potatoes keep their shape. Place sweet potato pulp in a large bowl. Set the halved and emptied sweet potatoes back on the baking sheet.
Add brown sugar, melted butter, sweetened condensed milk, cinnamon, nutmeg, and ginger in the bowl with the sweet potato pulp. Using a potato masher or hand mixer, blend ingredients together and smooth.
1/4 cup light brown sugar, 1/4 cup salted butter, 1 tablespoon sweetened condensed milk, 1 teaspoon ground cinnamon, 1/8 teaspoon ground nutmeg, 1/4 teaspoon ground ginger
Spoon blended sweet potato mix back into the sweet potato halves, filling each a little past full.
Place potato skins back in the oven and heat for 10-15 minutes, then remove from oven.
Set oven to broil.
For each potato half, press marshmallows in the soft sweet potato mix, completely covering the top.
3 cups miniature marshmallows
Place filled sweet potatoes back in the oven on the top rack until marshmallows melt and lightly brown, about 1-2 minutes. Since each oven is different, keep a close eye on the marshmallows so they don't burn. If they do burn, you can use two spoons to remove the marshmallows (they should come right up in a solid piece) then repeat the steps of placing the marshmallows on the sweet potatoes and lightly browning them in the oven.