1. Preheat oven to 350 degrees.
2. Line two standard muffin trays with liners (I used 6 “tulip” and 12 “standard” liners with this batch), and set aside.

For the Cinnamon Butter Crumble

1. In a small bowl, combine butter, oats, flour, brown sugar, and cinnamon with a fork until most of the ingredients form crumbs of various sizes. There will be some “sandy” ingredients at the bottom of the bowl, too.
2. Measure out the raw sugar (if using) and set aside.

For the Rhubarb Muffins

1. In a large mixing bowl, add the flour, brown sugar, oats, baking soda, and salt, and whisk to combine.
2. In a smaller, non-reactive mixing bowl, add the milk, vinegar, vanilla extract, oil, and egg, and whisk to combine.
3. Pour in the wet ingredients into the dry, and stir with a spatula just until combined.
4. Add 1¾ cup chopped rhubarb, and fold into the batter.
5. Fill muffin cups ½ - ¾ full with batter. Top with a few pieces of rhubarb – making sure to tuck them into the batter a bit. Sprinkle cinnamon butter crumble over the tops, and finish with a sprinkling of raw sugar (if using).
6. Bake in a 350 oven for 20 - 25 minutes, or until a toothpick inserted in the middle of a cupcake comes out fairly clean. Allow muffins to cool for 10 minutes, then remove from the tray and allow them to cool completely on a rack.
7. Store in an air tight container overnight, or refrigerate for up to 3 days. Or freeze up to 30 days.