In another bowl, cream the butter and brown sugar using an electric mixer. Add the eggs one at a time, beating until the mixture is smooth. Set the mixer to low speed and add the flour, alternating with the rum and molasses. Transfer the batter to a large bowl and add the remaining ingredients. Mix well, stirring with a spatula or wooden spoon for 4 or 5 minutes. Scrape the batter into the pans. Bake until a toothpick inserted in the centre comes out clean, about 90 minutes for smaller cakes or 2 hours for larger cakes.