Preheat oven to 350°F. Lightly grease a 13x9 inch casserole dish.
Peel the potatoes and cut them into 1–2 inch chunks. Place them in a saucepan and cover with water.
Place the pan over high heat and bring to a boil. Lower the heat to a simmer, and continue cooking the potatoes until they are tender, about 10–15 minutes. When they are tender, you should be able to easily slide a fork through the potatoes.
Drain the potatoes and place them in the bowl of a stand mixer. Alternatively, place them in a large bowl if you are using a hand mixer.
Add the butter and milk, and mix on low JUST until the mixture is blended and the potatoes are creamy.
Add the sugar, eggs, vanilla, and salt, and stir until combined. Pour the mixture into the greased 13x9 inch casserole dish.
For the pecan crumble:
Cut the butter into small cubes. Place the flour and brown sugar in a medium-sized bowl. Add the butter.
Use your fingers to rub the butter into the flour/brown sugar mixture until the butter is incorporated and crumbles form. Alternatively, you could use two forks or a pastry blender to incorporate the butter.
Stir the pecans into the crumble. Sprinkle the crumble on top of the sweet potatoes.
Bake for 30–35 minutes. The top will be golden brown. Serve while still warm.
Notes
You can substitute approximately 40 ounces of canned sweet potatoes for the fresh sweet potatoes. That's about 2–3 (15 oz) cans. However, make sure that the can does NOT say "in syrup" or "in light syrup". The can should say "cut sweet potatoes" or "sweet potato purée" and it should not contain any sugar. Read the label just to be sure! Make ahead instructions: prepare the sweet potato layer as directed, then cover and refrigerate. Prepare the pecan crumble but leave out the pecans. Place in the refrigerator. When ready to bake the casserole, stir the pecans into the crumble, then put the crumble on top of the sweet potato layer. Bake for 40–45 minutes. Do not put the two layers together until you're ready to bake the casserole. Reheating instructions: place the leftover casserole into a 350°F oven. Bake for 15–20 minutes, or until heated through. You can also microwave individual servings of the casserole, but the topping won't be as crunchy. Want to use marshmallows instead of pecans? Omit the pecan crumble. Top the sweet potato layer with one (10 oz) bag of mini marshmallows. Bake at 350°F for 25–30 minutes, or until the marshmallows are golden brown all over.