Rhubarb and Strawberry Upside-Down Cake

In a large bowl, sift together 1 cup all-purpose flour, 1-1/2 teaspoons baking powder and 1/4 teaspoon salt. Set aside. In another bowl, cream together 1/2 cup butter and 1 cup granulated sugar until fluffy. Add 2 eggs, 1/2 cup buttermilk and 1 ...