From Cleo Coyle's Coffeehouse Mysteries series. I made two batches of these, one using steel cut oats and the other using rolled oats. Both were good, just a different texture. I also got 9 muffins instead of 6 from both batches.
Combine buttermilk and oats, cover and refrigerate overnight or at least 6 hours.
Crack egg into a bowl and beat lightly with a fork. Add buttermilk/oat mixture, brown sugar, oil, cinnamon, vanilla, salt and raisins. Stir well to combine. Add flour, baking powder and baking soda and stir to create a lumpy dough.
Preheat oven to 375. Line muffin cups with paper liners and lightly coat the papers and top of pan with cooking spray.
Drop dough into cups, filling to the top. Bake 15 to 20 minutes, until top of muffin is firm to the touch and a toothpick comes out clean. Remove from pan and cool on a rack.