Preheat the oven to 400 F. In a large bowl, combine the all-purpose flour, whole wheat flour, brown sugar, baking soda, and salt.
Create a well in the middle of the dry ingredients, and pour in the buttermilk. Stir until combined, then mix the raisins into the dough.
Use your hands to lightly knead the dough, and shape it into a domed ball, making sure not to overmix.
Lightly brush the bottom of a 9-inch cast iron skillet with some of the melted butter. Then, place the dough ball into the skillet. Use a knife to cut about a ⅓ of the way through the dough in an X formation. Brush the top of the loaf with the remaining melted butter.
Bake the bread until the crust is golden, and the loaf is hollow when tapped, about 40 minutes. Allow the bread to cool completely, then slice and serve with butter or jam.