This ultra moist Clementine Cake is bursting at the seems with sunshiny mandarine citrus. Thinly sliced clementine oranges nestle into brown sugar and butter, topped with a buttermilk cake that has clementine zest and juice poured over top and baked until perfect. Invert onto a cake pan to reveal the beauty that is this homemade clementine cake. Serve with whipped cream if desired.Makes one 9-inch upside down cake.