Now:

Grease a 2-quart baking dish. In a large bowl, combine the sweet potatoes, sugar, eggs, milk, vanilla, and melted butter. Mix well, transfer to the prepared baking dish, and set aside.

In a small bowl, with a pastry blender, combine the flour, softened butter, and brown sugar, mixing until crumbly. Add the pecans, mix well, and sprinkle over the sweet potato mixture. Cover and refrigerate for up to 2 days.

Later:

Preheat the oven to 350°F. Bake, uncovered, for 35 to 45 minutes, until very lightly browned and crusty.