Beat butter and brown sugar in a stand mixer fitted with a paddle attachment until light and fluffy. Add in eggs, then molasses. Remember to scrape down the sides of bowl after each addition. Don’t worry if it seems curdled. Now alternate the flour mixture with the milk mixture. Beating on low, gradually add about a third of flour mixture. Beat just until mixed, followed by half the milk. Repeat additions, ending with flour. It will be very thick.