1. Butter a 9 by 13-inch baking dish or other similar shallow baking dish.

2. Scrub the potatoes, cut in 2- to 3-inch chunks, and boil in salted water until tender, about 20 minutes. Drain and when cool enough to handle, peel. Mash with a potato masher (you should have about 4 cups) and beat in the butter, sugar, milk, eggs, salt, and vanilla. Spread into the prepared dish.

3. In a food processor, pulse the pecans until chopped medium fine; remove. Add the brown sugar and flour to the workbowl and process to remove large lumps. Cut the butter into chunks, add to the workbowl, and pulse until the mixture resembles coarse meal. Add the pecans and pulse once or twice just to combine. (Or you can make this topping by hand.) Sprinkle over the sweet potatoes. (The casserole can be made several hours ahead and refrigerated. Return to room temperature before baking.)

4. Preheat the oven to 350°F. Bake the casserole until heated through and the topping is golden, 30 to 45 minutes.