Preheat oven to 350℉. In a mixing bowl, combing sweet potatoes, sugar, butter, eggs, vanilla, and evaporated milk until well incorporated.
Pour into a greased 13x9-inch baking dish.
For the streusel topping: In a small bowl, cut the butter into the brown sugar, baking powder, flour, and pecans until crumbly. Sprinkle evenly over top of sweet potatoes.
Bake 45 minutes or until casserole is set and topping is golden brown.