Dark brown sugar imparts a deep flavor that is reminiscent of molasses to this cake (and everything else it is used in making). Grated orange zest adds a bright note, and makes the cake appropriate for a winter dessert. Cake flour is finer than all purpose flour and results in a cake with a finer (or in other words softer) texture; be sure to use the kind that is not labelled as "self-rising" since baking powder is added here.