Preheat oven to 350°F (175°C) and butter an 8x8 inch baking dish.

In a medium pot, combine canned yams and their liquid. Bring to a gentle boil and simmer until fork-tender, about 10-15 minutes. Drain well.

Mash drained yams in a mixing bowl and add granulated sugar, melted butter, milk, eggs, and vanilla extract. Blend until creamy.

Spread sweet potato mixture evenly into the greased baking dish.

In a separate bowl, mix pecan pieces, brown sugar, flour, and melted butter until crumbly. Add on top of the sweet potato mixture.

Bake for 45 minutes, or until the topping is golden brown and casserole is bubbling.

Remove from oven and sprinkle mini marshmallows on top. Return to oven for an additional 5 minutes.

Let cool for about 15 minutes before serving.