Preheat your oven to 400°F (200°C).

In a large mixing bowl, combine the whole wheat flour, rye flour, brown sugar, baking soda, and salt.

Add the buttermilk and 1 cup of stout beer to the dry ingredients. Stir until the dough just comes together into a slightly sticky ball. If it seems too dry, add a splash more beer.

Turn the dough out onto a lightly floured surface. Gently shape it into a rough ball or football shape.

Place the shaped dough onto a prepared baking sheet. Sprinkle the top with a little flour.

Using a sharp knife, score a deep cross on the top of the dough.

Bake for 40-50 minutes, or until the loaf sounds hollow when tapped on the bottom.