Peel and chop sweet potatoes into 1 1/2 inch cubes. Add to a pot with cold water, then bring to a boil. Cook until potatoes are fork tender, about 20-30 minutes, then drain. While sweet potatoes are cooking, add butter to a small saucepan over medium heat. Whisking constantly, cook until little brown bits appear in the pan – about 4-5 minutes. Remove immediately from heat and set aside.
Preheat oven to 375 degrees F.
In the bowl of an electric mix (or a large bowl, using a hand mixer), add sweet potatoes. Add all remaining ingredients – sugar, syrup, spices, salt, vanilla extract, coconut milk and brown butter – and mix on low speed until combined about 30 seconds. Mix on medium speed for another 1-2 minutes, until potatoes are whipped. Spread in an oven-safe pan or dish. I used an 9 x 2 1/2 round tart pan.
Combine oats, flour, pecans, coconut, spices and salt in a bowl and mix to thoroughly combine. Add in softened butter, and use your hands to completely moisten the mixture and clump it together. Sprinkle the crumble around the outside of the dish, leaving room in the center for marshmallows. Add marshmallows to the middle. Bake for 20-25 minutes, or until crumble is golden and marshmallows are gooey.
Notes: If you want to skip the marshmallows, I suggest doubling the crumble mixture and covering the entire top with it. If you can’t find unsweetened, flaked coconut (I use Bob’s Red Mill brand), using sweetened is fine, but I would decrease the sugar by a 1/2 tablespoon (or more/less, depending on tastes). You can also sub in milk, cream or half and half for the coconut milk if you wish. However, the coconut milk does NOT add a coconut flavor to the casserole.