Use a tablespoon of the butter to grease a 9-inch square baking dish. Pre-heat oven to 350 degrees. Combine sugar and vanilla in a heavy saucepan with half a cup of water. Add plantain slices and cook over moderate heat until plantains are tender. Drain. The layer slices across the bottom of the baking dish. Set aside.
Combine flour, salt, baking soda and spices in a bowl. in a clean sauce-pan, bring butter and molasses just to a boil. Begin adding the flour/spice mixture and the milk a bit at a time, alternating them. Beat vigoursly. When all the remaining ingredients have been added to the pan, and mixture is smooth, pour over plantains in the baking dish. Bake 50 minutes to an hour, or until a knife inserted into the center comes out clean.