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

In a separate bowl, whisk together the buttermilk, egg, and softened butter until well combined.

Pour the wet ingredients into the dry ingredients and stir until just combined.

Turn the dough out onto a lightly floured surface.

Knead the dough until smooth.

Divide the dough in half.

Lightly press each half into a 1/2 inch thick round.

Transfer the rounds to a buttered large baking sheet.

Prick the tops of the rounds lightly with a fork.

Bake in a preheated oven at 425°F (220°C) for 20 minutes, or until golden brown.

Transfer the baked bread rounds to a wire rack to cool completely before slicing and serving.