Preheat the oven to 350°F.
In a large bowl, add sweet potato puree. Next add the milk, softened butter, cinnamon, eggs, ¼ cup of the brown sugar, granulated sugar, vanilla, and salt. Mix well.
Pour mixture in a casserole or baking dish; spread into an even layer.
Place flour, cold butter, the remaining brown sugar, and pecans into a bowl. Use a fork to stir ingredients together well.
Sprinkle the topping over the sweet potato mixture in the baking dish.
Bake for about 30 minutes until the top is golden and the mixture is warmed all the way through.
Scatter mini marshmallows in a single layer on top and put back in the oven for about 5 minutes. Watch the casserole closely as marshmallows can burn quickly!
Serve immediately, while still warm.