Preheat oven to 350F. Line muffin pan(s) with paper cups.
In a bowl, combine and mix flour, baking soda, baking powder, salt and 1 tsp cinnamon.
In two to three batches, fold flour mixture into wet mixture until barely combined. Add rhubarb and a stir gently, again until barely combined.
Batter will be very thick. Fill each muffin cup ¾ full with muffin batter.
In a small bowl combine melted butter, granulated sugar and ½ tsp cinnamon.
Bake for 20-25 until a toothpick comes out clean.