In a large bowl, combine flour, the granulated sugar, baking powder, cinnamon, baking soda and salt. Using a pastry blender, cut in the 1/2 cup butter till mixture resembles coarse crumbs. Stir in currants. Combine milk, sour cream and egg; add to flour mixture. Stir just to moisten. Spread the dough in prepared pan. Bake in a 450° oven for 15 to 20 minutes or till a wooden toothpick inserted near the center comes out clean. Cool in pan for 10 minutes. Remove from pan.