Using the largest bowl you have, combine the first four ingredients. Add the bran flakes and sugar, and mix. In a mixer bowl, beat the eggs. Add the oil, buttermilk, and vanilla to the eggs and blend. Pour the egg mixture over the flour mixture and stir well. Transfer the batter to a large plastic container that has a tight-fitting cover, and store in the refrigerator until ready to use. This batter will keep for 6 weeks. (Date the container the day you make the muffins) When ready to bake, do not stir batter when dipping out to fill the muffin pan. To bake, preheat the oven to 190ºC . Using about half cup batter for each, drop the batter into paper-lined muffin tins. Bake the muffins for 20 min, or until the top springs back when touched with your finger.