Heat oven to 200°. Grease 2 loaf pans with butter.
Whisk flours, baking powder, salt, and baking soda in a bowl.
Stir in buttermilk and syrup to form a smooth dough.
Pour dough into prepared pans and cover with aluminum foil; bake until cooked through, about 8 hours.
Let cool slightly and unmold; serve with butter, if you like.
