Mix first 6 ingredients. Add eggs, buttermilk, oil, and mix thoroughly. Cover batter and refrigerate overnight before using. (Spray muffin pans with non-stick spray) Bake at 200ºC or 20 minutes or at 180ºC or 25 minutes. Batter will keep in the refrigerator for 6 weeks. Muffins may be frozen after being baked.