Combine flour, salt and soda in a big bowl. Cut the butter into small cubes and crumble into dry ingredients. Mix in brown sugar or raisins, if desired. Add the buttermilk and stir well. Place on a floured board or other surface, and knead until it all stays together – it should not be too moist; adjust with flour or buttermilk as needed. Place in a round, lightly greased pan. With a floured knife, cut a large cross in the top. Bake at 350°F for 50 min. to one hour. If the top sounds hollow when you tap it, it is ready. Place on a rack to cool.