Cinnamon Topped Rhubarb Muffins

1/2 cup brown sugar, firmly packed
1/4 cup butter
1 (8oz) cup sour cream
2 eggs
1 1/2 cups all-purpose flour
3/4 teaspoon baking soda
1/2 teaspoon cinnamon
1 1/2 cups chopped rhubarb
1 tbsp sugar
1/2 teaspoon cinnamon

Heat oven to 375 F. In large bowl combine brown sugar and butter. Beat at meduim speed until well mixed (1-2 min). Add sour cream and eggs; continue beating ; scraping bowl often, until well mixed (1-2 min) In medium bowl, stir together flour, baking soda and 1/2 teaspoon cinnamon. By hand, stir flour mixture into sour cream mixture just until moistened. Fold in rhubarb. Spoon into greased muffin pans. In small bowl, stir together 1 tbs sugar and 1/2 teaspoon cinnamon, sprinkle onto each muffin. Bake for 25 to 30 min or until lightly browned. Cool 10 min; remove from pans.