Preheat oven to 350°. In a large bowl, cream butter and sugars until light and fluffy, 5-7 minutes. Beat in egg. Combine flour, baking powder, baking soda and salt; add to creamed mixture alternately with sour cream, beating well after each addition. Fold in rhubarb. , Fill paper-lined or greased muffin cups three-fourths full. For topping, combine the pecans, brown sugar and cinnamon in a small bowl; cut in butter until crumbly. Sprinkle over batter. , Bake until a toothpick inserted in the center comes out clean, 22-25 minutes. Cool for 5 minutes before removing from pans to wire racks. Serve warm.