Transfer Sweet Potato (10) to a large pot and submerge them in cold water. Bring the pot to a boil and boil the sweet potatoes until they are fork tender for about 15 minutes or so. Drain them and dump them into a large mixing bowl.
Add theButter (1/4 cup) and lightly mash everything together with a fork. Then add the Dark Brown Sugar (10 tablespoon), Ground Cinnamon (1 teaspoon), Ground Nutmeg (1/2 teaspoon), Salt (1 pinch), Milk (1/2 cup) and Egg (2). Use a hand mixer to whip them together thoroughly and make the mixture fluffy.
Preheat the oven to 350 degrees F (180 degrees C) and get out a 9 x 13 baking dish. Spray it with cooking oil.
Turn out the sweet potato mixture into the pan and spread it out evenly, smoothing out the top. Then make the easy topping. Stir the Walnut (1 cup), Dark Brown Sugar (2/3 cup), Ground Cinnamon (1/2 teaspoon), Salt (1 pinch), Butter (5 tablespoon), and Vanilla Extract (1/2 teaspoon) together thoroughly in a bowl until it is a sticky glaze. Evenly pour it over the sweet potato mixture, using a spoon to spread it out thinly.
Bake the casserole for 45 minutes, until completely set. Let it cool for a couple of minutes, then serve immediately! It can also be assembled ahead the day before, covered and then baked the day of.