Preheat the oven to 425F/220C. Lightly sprinkle flour around the inside of the cast iron pan.
In a large bowl, whisk together the flour, baking soda, and salt.
Add the buttermilk and mix gently with a spoon until the dough is just combined but still flaky.
On a lightly floured surface, knead the dough 8-10 times. Then, place the dough in the prepared cast iron pan and carefully flatten to about 2 inches of thickness. Use a knife to cut the shape of an X into the surface of the dough at about ½ inches deep.
Bake the bread at 425F/220C for 15 minutes.
Lower the heat to 375F/190C and bake for an additional 20 minutes or until the dough is slightly golden in color.
Slice and serve while warm with optional butter, as desired.