Preheat the oven to 350°F (175°C) and arrange an oven rack in the middle of the oven. Butter or spray a 9x13-inch casserole or baking dish.

Spread 1 cup of chopped pecans on a baking sheet and roast in the preheating oven until fragrant and lightly toasted (about 6–8 minutes). Watch closely to avoid burning.

Place the peeled and cubed sweet potatoes in a large pot and cover with cold water. Bring to a boil, then reduce heat and simmer until very tender, about 15–20 minutes.

Drain the sweet potatoes thoroughly and return them to the pot or to a large bowl. Mash the sweet potatoes until smooth using a potato masher or ricer.

In a small bowl, mix the 1/4 cup sugar (or light brown sugar) with the 1/2 teaspoon ground cinnamon for the topping. Stir in the 1/4 cup melted butter and the roasted pecans until evenly coated.

In a large bowl, combine the mashed sweet potatoes with the 1/2 cup light brown sugar, lightly beaten eggs, 1 cup cream (or whole milk), 1/2 cup self-rising flour, 1 teaspoon vanilla extract, 1/2 teaspoon salt, and 1/2 cup melted unsalted butter. Mix gently until just combined—avoid overmixing.

Spread the sweet potato mixture evenly in the prepared 9x13-inch baking dish.

Sprinkle the pecan topping evenly over the sweet potato layer.

Bake for 30–40 minutes, or until the center is set and the topping is golden and crisp.

Remove from the oven and allow the soufflé to rest for 5–10 minutes before serving.