Preheat the oven to 375°F conventional (no fan) or 350°F convection (with fan). I used a conventional oven set to 375°F to avoid overly drying out the crust with the fan. 

In a large bowl or in the bowl of a stand mixer fitted with the paddle attachment, toss together the wheat bran, whole wheat flour, all-purpose flour, sugar, baking powder, baking soda, and kosher salt. 

Add the cubed cold butter and either cut it in with a pastry blender or with a stand mixer until very few pieces remain; none should be larger than a grain of rice. 

If using, remove the bowl from the stand mixer and stir in the buttermilk with a fork. Turn out the dough onto a smooth surface like a kitchen counter or a large wooden cutting board. 

Gently knead the dough into a round ball. 

Place the dough onto a parchment-lined baking sheet or in a cast iron skillet. Using a sharp knife, cut a crisscross into the top of the dough, being sure to cut ½ inch deep. This allows for a controlled, even rise. 

Bake in the preheated oven for 40-45 minutes or until a skewer or cake tester comes out clean from the center. I rotated my loaf after 15 minutes. 

Remove to a wire rack to cool completely before slicing.