1. Preheat the oven to 350°. In a medium saucepan, combine the sugar and water. Bring to a boil, then reduce the heat and simmer until the water has reduced slightly, about 5 to 7 minutes. Add the pecans and stir vigorously with a wooden spoon. Add a pinch of salt; using a spoon, transfer the pecans onto a baking sheet. Set aside to cool to room temperature.

2. Place the sweet potatoes on a baking sheet and rub with the oil. Season with salt and white pepper and place in the oven. Cook until the potatoes are tender, about 1 hour. Remove the potatoes and set aside to rest until they're cool enough to handle, about 7 to 10 minutes. Carefully peel off the skins.

3. In a food processor, combine the sweet-potato flesh, butter, vanilla extract and cane syrup and purée until smooth. Season with salt and white pepper. Spoon the mixture into an 8- or 9-inch round casserole dish and top with the marshmallows. Use a torch to caramelize the marshmallows. (Alternatively, turn on the broiler and broil the marshmallows for a few minutes until they've caramelized.) Top the casserole with the reserved candied pecans and serve immediately.