If baking casserole immediately, preheat oven to 375 F.
Place potatoes in a large pot and add water to cover them by about 1 inch. Bring pot to a boil, then lower heat to a simmer and partially cover the pot. Cook potatoes for 15-20 minutes or until soft and starting to fall apart.
While potatoes are cooking, make streusel topping. Mix melted butter with flour, brown sugar and cinnamon (if using) and stir to combine until small clumps form. Gently stir in pecans (if using). Set aside. (I like to use the same pot that I melted the butter in to make the streusel topping so I have one less thing to wash.)
When the potatoes are cooked, drain them well in a colander. Set aside.
Combine the half and half, maple syrup, vanilla, salt, egg and bourbon in a large bowl with a whisk until smooth. Add drained sweet potatoes. Beat with a hand mixer at medium speed until smooth.
Spray a baking dish (9x11 or 9x13) with cooking spray or grease with butter. Spoon potato mixture into dish and spread evenly, smoothing the top with a spatula. Sprinkle the streusel topping evenly over the potatoes.
Allow to cool, and then cover with plastic wrap and refrigerate for up to 3 days. For longer storage, you can cover the wrapped casserole with tin foil and freeze for up to one month. Thaw overnight in the refrigerator and bring to room temperature before baking.
Before baking, remove plastic wrap from casserole. Cover with tin foil and bake at 375 F for 15 minutes. Uncover and bake for an additional 15-30 minutes until the casserole is heated through and the topping is brown and crispy.