Combine the flour, baking soda, salt and brown sugar in a bowl; mix well. Stir in the peanut butter, egg and buttermilk.
Spoon the batter into a 5x9-inch loaf pan.
Bake at 325 degrees for 40 to 45 minutes or until the loaf tests done. Invert onto a wire rack.
Variation: Add 1/2 cup raisins or chocolate chips.
NOTE: The flavor of the bread is best when served warm.