Preheat oven to 375°. Grease four 4-1/2 x 2-1/2 x 2 -1/2" loaf pans. Place the bran cereal in a small bowl and cover with the buttermilk; let stand until needed.
Combine the dry ingredients in a medium bowl. In a separate bowl, beat the butter until creamy; beat in the sugar until well blended. Add the egg; beat until light and fluffy. Beat in the flour mixture in three additions and the buttermilk-bran mixture in two, beginning and ending with the flour mixture.
Divide the batter evenly among the prepared pans; smooth the tops with a spatula. Place on a baking sheet; bake until the loaves are lightly browned and a wooden pick inserted in the centers is withdrawn clean, about 15 minutes. Remove the loaves from the pans and cool on a rack.