Poke holes all over the sweet potatoes with a fork and place them on a large microwave-safe plate. Microwave on high until the potatoes are very fork-tender, 15-20 minutes. Carefully remove the plate from the microwave and allow the sweet potatoes to cool until cool enough to handle, about 15 minutes. Microwave in batches if necessary.

Preheat the oven to 375°F.

Spray a 9x13-inch baking dish with nonstick cooking spray. Set aside.

Scoop the baked sweet potato flesh from the skins into a large mixing bowl. Discard the skins. Add the brown sugar, butter, cream or half and half, pumpkin pie spice, salt, and pepper. Mash the mixture with a potato masher or hand mixer until it is smooth and creamy. Add the beaten eggs to the bowl and stir to combine until smooth.

Transfer the sweet potato mixture to the prepared baking dish and smooth it into an even layer. Sprinkle the marshmallows evenly over the top.

Bake the casserole on middle rack of oven until the marshmallows are puffed and golden, and the dish is gently bubbling at the edges, 30-35 minutes.

Check to see that the casserole is done. Remove from oven or add time as needed.

Allow the sweet potato casserole to cool for 5-10 minutes before serving.