Preheat your oven to 375°F (190°C). Grease a 12-cup muffin tin with non-stick cooking spray or line it with paper muffin liners to ensure easy removal after baking.

In a large mixing bowl, combine the wheat bran and buttermilk. Let the mixture sit for about 10 minutes. This allows the bran to soften, which is the secret to a moist and tender muffin texture.

In a separate medium bowl, whisk together the egg, vegetable oil, brown sugar, and vanilla extract until the mixture is smooth and the sugar has mostly dissolved.

Pour the egg and sugar mixture into the bowl with the soaked bran. Stir well with a spatula or wooden spoon until the ingredients are fully incorporated.

Sift the all-purpose flour, baking soda, baking powder, and salt directly into the wet ingredients. Fold gently until just combined. Be careful not to overmix, as this can make the muffins tough. If using raisins, fold them in now.

Spoon the batter evenly into the prepared muffin tin. Place in the center rack of the oven and bake for 18 to 20 minutes. A toothpick inserted into the center should come out clean.

Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely. This prevents the bottoms from becoming soggy from steam.