Preheat the oven to 300°F (150°C). Grease a 9×5-inch loaf pan well and set aside.
In a large bowl, mix together the whole wheat flour, all-purpose flour, salt, baking soda, baking powder and brown sugar.
Measure the buttermilk into a large measuring cup. Alternatively, add 1 1/2 tablespoons vinegar or lemon juice to the measuring cup, then add milk to the 2-cup line.
Stir the molasses into the buttermilk until well combined, then pour into the dry ingredients. Stir until thoroughly combined.
Pour the batter into the prepared loaf pan and let it stand for 10 minutes. This short resting time allows the whole wheat flour to hydrate before baking and is part of the traditional method for this family recipe.
Bake for about 60 minutes or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Remove from the oven and, if needed, run a knife around the edges of the loaf to loosen it from the pan. Turn the bread out onto a wire rack and let cool for at least 30 minutes before slicing. Serve warm with butter, if desired.
Store the cooled bread in an airtight container at room temperature for up to 4 days. For the best texture and flavor, serve at room temperature or warm individual slices briefly before serving. To freeze, wrap the cooled loaf or individual slices tightly and freeze for up to 3 months. Thaw at room temperature before serving.
