Boiled cubed sweet potatoes in an inch of water for 15 minutes or until very tender. Water should mostly be absorbed.
Add butter, vanilla extract, and milk. Mash or whisk sweet potatoes until smooth.
Transfer to an 8-inch diameter casserole dish.
In a medium size bowl, combine all-purpose flour, brown sugar, butter, and cinnamon. Cut butter with fork until it resembles coarse breadcrumbs. Stir in pecans.
Sprinkle topping over sweet potato mixture.
Bake in a preheated 350˚F (180˚C) oven for 30 minutes or until heated through and topping is lightly brown.