Preheat the oven to 375°F. with the rack in the center position. Spray a muffin pan with nonstick vegetable cooking spray and line each tin with a paper or foil liner. Or grease the pan with butter and flour it, knocking out the excess flour.
In a large mixing bowl combine the egg, milk, butter or margarine, and bran flakes. Mix and let stand for 10 minutes while you prepare the other ingredients.
Sift the flour, baking powder, and salt over the mixture in the bowl. Add the sugar and raisins and mix just enough to combine the ingredients. Spoon the batter into the prepared pan, filling each cup two-thirds full. Bake for 20 minutes, or until a cake tester or a toothpick inserted in the middle of a muffin comes out clean and dry. Cool the muffins on a rack, or tilt them on their sides in the pan to cool.
Can be made ahead? Yes. The day before. Cool, then store in an airtight plastic bag at room temperature.
Can be frozen? Yes. Cool completely, and store in airtight freezer bags. Defrost at room temperature and warm in a 300°F. oven or toaster oven for 5 to 10 minutes.
Can be doubled? Yes, if you have several muffin pans.
Good for leftovers? Muffins will keep 2 to 3 days stored in a plastic bag at room temperature and up to 4 days, refrigerated.