• Preheat oven to 400 degrees F. Line a baking sheet with foil or parchment paper. Place potatoes on the sheet and bake for 45 minutes to an hour, until potatoes are tender. Let cool before peeling.
• Reduce oven temperature to 350 degrees F. Spray a 9 x 13 pan with nonstick spray.
• Peel off the potato skin (using your hands, it comes off easily) and put potatoes in a large bowl. Add the softened butter, milk, brown sugar, vanilla, salt, and eggs. Mash until smooth. (Use an electric mixer if you want the texture to be really smooth.) Spoon the sweet potato mixture into the prepared baking dish and smooth out.
• In a small bowl, use a small rubber spatula to mix together the melted butter, brown sugar, flour, oats, and cinnamon. Crumble over sweet potatoes.
• Bake for 20-25 minutes, until lightly golden brown on top. Let cool slightly.
• Serve and Enjoy!

Make ahead:

• This can easily be made ahead of time. You can bake the sweet potatoes 1-2 days before and once cool, keep them stored in the fridge in an airtight container. (Ziplocks work great.)
• Also, the sweet potato mixture and oat streusel can be prepared but instead of baking, cover tightly and store in the fridge, up to 2 days. Bring to room temperature. Add about 5-10 minutes longer to the bake time.
• Assembled and baked casserole can be frozen up to 2 months. Thaw overnight in the refrigerator, bring to room temperature and then bake according to directions in the recipe, adding 5-10 minutes to baking time if needed.