Preheat your oven to 375°F (190°C). Grease a muffin tin or line with paper liners.
In a large bowl, combine the bran cereal and buttermilk. Let sit for 10 minutes.
Add the vegetable oil, egg, brown sugar, and vanilla extract to the bran mixture. Mix well.
In another bowl, whisk together the flour, baking soda, and salt. Gradually add to the wet ingredients, mixing until just combined.
Fold in the raisins if using. Divide the batter evenly among the muffin cups.
Bake for 20 minutes, or until a toothpick inserted into the center comes out clean. Let cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.
Notes:
These muffins can be stored in an airtight container for up to 3 days or frozen for up to 3 months.