Pre-heat oven to 350 degrees. Line 2-12 spot muffin pans with muffin papers or spray with baking spray. (These muffins are pretty sticky even when cooked so I like to use muffin/cupcake papers).
In a large bowl, stir together melted butter and sugar.
Stir in milk, egg, baking soda, vanilla extract and cinnamon.
Gently stir in both flours. Fold in chopped rhubarb.
Drop tablespoonfuls of batter into prepared muffin pans. Sprinkle topping over muffins.
Bake at 350 degress for 18-22 minutes.