In a mixing bowl, cream butter and sugars. Add egg; beat well. Combine the flour, baking powder, baking soda and salt; add to creamed mixture alternately with sour cream. 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 at 350 for 22-25 minutes or until a toothpick comes out clean. Cool for 5 minutes before removing from pans to wire racks.