In a large bowl, combine the flour, baking powder, baking soda and salt and mix well. Cut the butter and shortening into small pieces and add to the flour mixture. Using your fingers, work the cold butter and shortening into the flour mixture until it resembles coarse crumbs. Add the egg, the buttermilk, the currants and the caraway seeds and mix into the flour mixture until it is incorporated. Turn the dough out onto a floured surface and knead gently until the dough forms a smooth ball. Place the dough into a lightly greased loaf pan. Score the top of the loaf with a very sharp knife. Brush the top of the loaf with melted butter. Sprinkle with sanding sugar. Place the loaf in the oven and bake for 40 to 45 minutes or until golden brown.