Combine the cereal, flour, brown sugar, baking soda, salt, and cinnamon into a large bowl and whisk together. Set aside.
In a separate bowl whisk together the buttermilk, oil, egg, and vanilla. Pour the wet ingredients into the dry and stir to blend well. Cover the bowl tightly with plastic wrap and refrigerate for at least 4 hours. The batter will become very thick
Preheat oven to 400°F. Lightly grease your muffin tin and set aside.
Divide the batter evenly among the muffin cups.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean. Transfer muffins to a wire rack to cool completely.
Note – Measure 3 cups of the the Bran Cereal and THEN crush it. You won’t have a total of 3 cups of crushed cereal. The measurement is for pre-crushed.