A birthday cake needn't be elaborate. A few layers of tender yellow cake and creamy chocolate frosting will do the trick. In this version of the classic pairing, brown sugar and buttermilk provide a sophisticated flavor to the cake, and sour cream adds a slight tang to the chocolate frosting. It’s worth noting that both the cake and frosting can be made ahead. Just make sure you bring the frosting to room temperature before assembly so that it spreads easily. One note: The buttermilk and…