Preheat your oven to 375°F (190°C) and lightly grease a baking dish.

Boil the diced sweet potatoes in a pot of lightly salted water until they are tender, which typically takes 15-20 minutes. Then, drain the water thoroughly.

Transfer the cooked sweet potatoes to a large bowl and mash them well. Add 1/2 cup brown sugar, 1/4 cup melted butter, 1/4 cup milk, 1 teaspoon vanilla extract, 1/2 teaspoon cinnamon, and 1/4 teaspoon salt to the mashed sweet potatoes, then mix until everything is combined and smooth.

In a separate small bowl, combine the chopped pecans, 2 tablespoons flour, 2 tablespoons brown sugar, and 2 tablespoons melted butter. Mix these ingredients until they form a crumbly topping.

Spread the sweet potato mixture evenly into the prepared baking dish. Sprinkle the pecan topping over the sweet potatoes.

Bake for 25-30 minutes, or until the topping is golden brown and the casserole is heated through. Serve warm.