Preheat oven to 140 degrees C. Butter a 1 litre baking dish.
In a large saucepan over medium heat, heat milk until bubbles form at edges. Slowly stir polenta and molasses into milk. Continue to cook and stir until thickened, about 10 minutes. Remove from heat and stir in sugar, salt, ginger and cinnamon. Pour into prepared dish.
Bake in preheated oven 2 hours until set. Serve hot or cold.