Preheat the oven to 375˚. In the bowl of a mixer, combine the flour, sugar, baking soda, and salt. Using your fingers, work 4 tablespoons cold butter into the flour until the butter is in small bits.  In a separate small bowl, whisk the egg into the buttermilk and pour into the flour mixture. Using the paddle attachment, mix on low speed for a few turns, until the dough is barely incorporated; there will still be some dry flour in the bowl. Add the raisins and cranberries and bring the dough together with your hands, kneading until the dough comes together and there aren’t any dry bits of flour left.  Form the dough into a round and place in a medium-size cast-iron pan. With a sharp knife, cut a large X on the top of the dough (this will help it bake through). Bake for 50 to 55 minutes, until the top is golden brown and the bread sounds hollow when you tap on it.  Melt the remaining 4 tablespoons butter. Remove the pan from the oven and brush the top of the bread with the melted butter.