Heat oven to 375F. Mix flour through nutmeg in a bowl and set aside.

Beat the butter until light and fluffy, 1 to 2 minutes. Add the brown sugar and beat until combined and fluffy, about 1 minute more. Add eggs one at a time, beating well. Beat in vanilla, molasses and sour cream, and beat 1 minute more. Add the buttermilk and half of the flour mixture. Beat in the remaining flour mixture until incorporated and slightly curdled looking. Stir in bran and raisins.

Pour batter into greased muffin tins. Bake until a toothpick inserted in the center of a muffin comes out dry or with a few moist crumbs attached, about 25 minutes. Do not overbake! Cool on a rack 5 minutes and turn out to finish cooling.