1. Preheat oven to 375 F. Place rack in center of oven. Line a 12-cup muffin or cupcake tin with paper liners.
2. In large bowl, combine flour, sugar, baking powder, cinnamon, baking soda and salt. Whisk to blend.
3. In medium mixing bowl, add sour cream, melted butter, eggs and vanilla. Whisk together until smooth.
4. With a spatula, carefully fold sour cream mixture into dry ingredients just until batter comes together. Batter will be thick. Do not over mix.
5. Gently stir in diced rhubarb.
6. Use measuring cup or small ice cream scoop to fill muffin cups about 3/4 full.
7. Make the cinnamon sugar topping (optional). In small bowl, mix together the sugar and cinnamon. Sprinkle over each muffin.
8. Bake for about 20 minutes. Turn pan about half way through baking cycle. When done, muffins tops should be golden brown and a toothpick inserted in center should come out clean.
9. Let muffins rest in pan for about 5 minutes. Remove muffins to cooling rack. Serve warm. Makes 12 to 18 medium-sized muffins.