NOTE: This mix is made the day before you want the muffins. You can bake them sooner, but the overnight time in the fridge gives the bran time to soften and thicken. So make the mix the day before and in the morning all you have to do is bake. (Another secret of the innkeeping game...!)
In a large bowl whisk together the flour, baking powder, cinnamon, baking soda, salt and bran.
In another bowl, mash up the banana. Add the eggs, oil, brown sugar and molasses. Whisk the mixture until the ingredients are emulsified. (They all come together into a thick-ish liquid.) Then mix in the buttermilk and vanilla.
Add the liquid ingredients to the dry ingredients and mix only until everything is combined. Add the currants and stir to distribute.
Cover the bowl and refrigerate overnight.
When you are ready to bake - preheat the oven to 375F. Grease or line about 16 muffins cups. (The original recipe says it makes 12 standard muffins - I always end up with 16.) Fill the muffin cups and sprinkle with the raw sugar.
Bake for about 24 minutes.