Peel and slice the sweet potatoes. You should have about 3 cups. Bring a medium saucepan of water to a boil. Add the sweet potatoes. Reduce the heat to medium-low and cook for about 8 minutes, or until the sweet potatoes are just tender. They should be tender but not falling apart. Drain and set aside to cool.

Heat the oven to 350 F.

Put the butter in a 2-quart baking dish and place it in the oven until the butter has melted.

Meanwhile, in a bowl, combine the flour, 1 cup of sugar, and milk; mix until well blended.

Combine the remaining 1 cup of sugar with the brown sugar, water, vanilla, and cinnamon. Mix well and add the sweet potatoes.

Take the baking dish with the melted butter out of the oven. Pour the batter into the center of the melted butter; do not stir.

Gently spoon the sweet potatoes and their liquids into the center of the batter; do not stir.

Place the baking dish on a rimmed baking sheet to catch possible spills and bake for about 45 minutes or until golden brown.