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 it sit for about 10 minutes to soften.
In another bowl, whisk together the oil, egg, and brown sugar until well combined.
Add the wet ingredients to the bran mixture and stir to combine.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
Add the dry ingredients to the wet mixture and stir until just combined. Do not overmix.
Fold in the raisins.
Divide the batter evenly among the muffin cups.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.