Soften butter to room temperature. Preheat oven to 350 degrees F.

Mix sour cream and baking soda together in a medium bowl and set aside.

Cream butter or margarine and brown sugar together. Add eggs, one at a time, beating well after each addition.

Combine bran, flour, salt, and baking powder.

Add dry ingredients and sour cream alternately to the creamed mixture.

Fill muffin tins, lined with paper muffin cups or greased, 2/3 full.

Bake for 20 - 25 minutes, or until tops spring back when touched.