Preheat oven to 350. Line muffin try w/ muffin liners or grease the cups instead.

In a medium bowl combine flour, baking soda, salt and sugar.

In another bowl, beat the egg, stir in butter, buttermilk and vanilla.

Make a well in the dry ingredients, and add the wet mixture all at once. Stir until just blended; stir in the chopped rhubarb.

Spoon into greased or paper lined muffin tins.

Topping:.

For the topping: Combine the 2 tbsp melted butter, brown sugar and cinnamon.

Sprinkle a spoonful of topping over each muffin.

Bake at 350 degrees for 20 to 25 minutes or until toothpick tests them done.