Buttermilk is acidic, and baking soda is alkaline. The reaction of these two ingredients provides leavening in this recipe.
It is important not to overmix quick breads and muffins; overmixed batter produces tough baked goods with peaked tops.
Bake these muffins early in the day and let them stand, loosely covered, at room temperature. Reheat them at 375°F for 5 to 7 minutes.