Step 1: Prepare the sweet potatoes

• Preheat the oven to 375°F (190°C).
• Place the cubed sweet potatoes in a large pot of salted water. Bring to a boil and cook until tender, about 15–20 minutes. Drain and transfer to a large mixing bowl.

Step 2: Make the sweet potato base

• Mash the sweet potatoes until smooth. Add softened butter, brown sugar, heavy cream, vanilla extract, eggs, salt, and cinnamon. Mix until fully combined and creamy.

Step 3: Transfer to the baking dish

• Spread the sweet potato mixture evenly into a greased 9×13-inch baking dish.

Step 4: Prepare the streusel topping

• In a medium bowl, combine flour, brown sugar, cinnamon, and cold butter. Use a pastry cutter or your fingers to mix until the mixture resembles coarse crumbs. Stir in the chopped pecans.

Step 5: Add the topping

• Sprinkle the streusel evenly over the sweet potato mixture. For the marshmallow option, scatter the mini marshmallows on top instead of the streusel, or use both for extra indulgence.

Step 6: Bake the casserole

• Bake in the preheated oven for 25–30 minutes, or until the topping is golden brown and the marshmallows are melted and lightly browned if using.

Step 7: Cool and serve

• Let the casserole cool for 5–10 minutes before serving.