In a small bowl, combine bran and milk; let stand for 10 minutes. Combine dry ingredients and set aside.
In a bowl, cream butter and brown sugar until light and fluffy. Add the egg, potatoes and bran mixture; beat until smooth. Stir in dry ingredients and raisins just until moistened. Spoon into 12 greased muffin cups.
Bake at 375° for 20 minutes or until a toothpick inserted in the center comes out clean.