Adjust oven rack to lower middle position and heat oven to 375 degrees.
Prepare a 12 (1/2 cup) or 6 (1 cup) muffin tins with paper or silicon cups. Alternatively you can lightly grease with butter.
Mix flour through nutmeg in medium bowl; set aside.Beat butter in large bowl of electric mixer or with handheld mixer at medium speed until light and fluffy, 1 to 2 minutes. Add brown sugar, increase speed to medium-high, and beat until combined and fluffy, about 1 minute longer.
Add eggs one at a time, beating thoroughly before adding the next. Beat in vanilla and molasses until thoroughly combined and creamy, about 1 minute longer.
Reduce speed to low; beat in buttermilk and half the flour mixture until combined, about 1 minute. Beat in remaining flour mixture until incorporated and slightly curdled looking, about 1 minute longer, scraping sides of bowl as necessary. Stir in bran and raisins.
Divide batter evenly among molds using spoon or ice cream scoop. Bake until a toothpick inserted into center withdraws cleanly or with a few moist particles adhering to it, about 25 minutes.
Set on a wire rack to cool slightly, about 5 minutes. Remove muffins from tin and serve warm.
Notes
*you can make your own buttermilk by adding a splash of lemon juice or vinegar to your measured milk and let it sit for 5 minutes until the milk curdles.