Preheat oven to 350 degrees F (175 degrees C). Grease an 8-inch square baking dish.
Put yams in a microwave safe bowl; heat in the microwave oven until softened, about 75 seconds.
Transfer cooked yams to a large mixing bowl; add white sugar, milk, 1/2 cup butter, and eggs and beat with a hand mixer until smooth. Pour the yam mixture into the prepared baking dish.
Mix brown sugar, pecans, 1/2 cup butter, and flour together in a bowl with a wooden spoon until crumbly, sprinkle over the yam mixture to cover.
Bake in preheated oven until golden brown, about 30 minutes.