In a large bowl with an electric mixer cream together the butter and the brown sugar until the mixture is light and fluffy, beat in the egg, the sour cream, and the molasses, and stir in the raisins. In a bowl whisk together the flour, the baking soda, the salt, and the bran, add the mixture to the sour cream mixture, and stir the batter until it is just combined. (The batter will be lumpy.) Spoon the batter into 12 well-buttered 1/3-cup muffin tins and bake the muffins in the middle of a preheated 400°F. oven for 15 to 20 minutes, or until they are golden brown and springy to the touch. Turn the muffins out onto a rack and let them cool.