Preheat oven to 350°. Melt butter on stovetop in a heavy skillet, preferably cast iron. Add brown sugar and cook over medium heat, stirring until the sugar has dissolved. Add rhubarb and stir to coat. Remove from heat and sprinkle on dried cherries, if using. Set aside. In a bowl, sift together flour, baking powder and salt. In another bowl, cream together butter and sugar with beaters until fluffy. Add eggs, buttermilk and vanilla and beat in well. Stir in flour mixture until thoroughly blended. Pour batter over the rhubarb in the skillet and bake 30–40 minutes until done. Carefully invert the pan onto a serving plate and let sit for a minute to allow the juices to run down. Remove the pan and serve warm.