On a foil-covered baking sheet, bake the sweet potatoes for 1 hour, or until they are soft. Remove and let cool. Reduce the heat of the oven to 325°F. When cool enough to handle, peel the potatoes, place the flesh in a large mixing bowl, add the butter, and mash until smooth. Add the eggs, one at a time, beating well after each addition. Add the cornmeal, sugar, milk, salt, and ½ teaspoon of the vanilla and beat until smooth. Pour the batter into the prepared baking dish. Arrange the pecan halves on top of the pudding and top with the brown sugar. Sprinkle the remaining ½ teaspoon vanilla by small droplets over the brown sugar. Bake for 45 minutes, or until the top is browned.