Combine 1 cup wheat bran and 1 cup boiling water; stir and let steep. In separate bowl, cream margarine and brown sugar together. Combine flour, salt, baking soda in a large mixing bowl. Stir in the 1 cup of steeped bran with remaining 2 cups of wheat bran, eggs, buttermilk, margarine and sugar. Mix well.
Store in tightly covered plastic container. Let stand at least 12 hours before baking. Batter will keep in refrigerator 6 weeks; stir before spooning into muffin tins. (Mixture will turn dark on top upon standing; no problem, just stir before spooning into muffin tins.) Fill muffin tins 2/3 full; bake at 400 F for 20 minutes.