Preheat oven to 400 degrees. Lightly grease a 12-cup muffin tin or line with muffin papers.
In a large bowl, combine the flour, sugar, baking powder, baking soda, cinnamon, and salt. Stir in rhubarb.
In a medium bowl, whisk together the sour cream, melted butter, eggs, and vanilla until smooth. Lightly stir the wet mixture into the dry ingredients, just until the batter comes together and it is fully incorporated. Do not overmix.
Divide batter over 12 muffins, smoothing tops slightly with back of spoon.
For topping, stir together sugar and cinnamon and sprinkle a generous ½ tsp over each muffin.
Bake for 18-20 minutes, until golden and toothpick comes out clean. Let cool in pan 5-10 minutes before removing to racks to cool completely.
Notes
These muffins are best served slightly warm - eat them fresh, and freeze any leftovers once they are cooled. Reheat as needed for 30 seconds in the microwave.
Recipe by Hidden Ponies at https://hiddenponies.com/2012/04/cinnamon-rhubarb-muffins/