Place sweet potatoes in a pot and cover completely with water. Bring to a boil; allow to cook until potatoes are soft (10-15 minutes).
Place cooked potatoes in bowl along with butter; allow butter to melt, and then mix to combine ingredients.
Add the remainder of your casserole ingredients to the potatoes; mix thoroughly to combine. Once the potatoes have been mixed to a smooth consistency, transfer to a greased 8" by 8" baking dish. Preheat oven to 350 degrees F.
While the oven is preheating, make the streusel topping. First, mix together the melted butter and brown sugar. Then add the flour, pecans, and spices and mix thoroughly. The final texture should resemble wet sand.
Pour topping over the potato mixture, spreading in an even layer. Bake for 30 minutes at 350 degrees F.
Allow to cool for 5-10 minutes before serving.