Preheat the oven to 425°F. Coat a 13- x 9-inch baking dish with nonstick cooking spray. Set aside.
Bake the sweet potatoes on a rimmed baking sheet for 45 minutes or until very tender. (Use the tip of a knife to pierce the sweet potato. If it goes through easily, the potatoes are done.) Peel the potatoes and mash in a large bowl.
Whisk together the half and half, vanilla, cinnamon, egg, brown sugar, and salt. Add to the sweet potatoes and stir until the mixture is smooth. Transfer the mixture to the prepared baking dish.
Make the streusel topping: Combine the pecans, butter, brown sugar, and salt in a medium bowl. Squeeze the mixture into large clumps.
Reduce the oven temperature to 350°F. Crumble the streusel over the sweet potatoes. Cover the casserole with aluminum foil and bake for 20 minutes.
Remove the foil and bake for 15 more minutes or until the streusel is toasted and the casserole is bubbly.