Preheat oven to 350°. Line a muffin tin with paper lines or grease with butter or oil.
In a large bowl, cream butter and sugars until light and fluffy. Beat in egg. In a separate bowl, combine flour, baking powder, baking soda and salt. Add dry mix to the wet mix, and then beat in sour cream. Mix well and then fold in rhubarb. Fill muffin cups ¾ full with batter.
To make the streusel topping, combine the pecans, brown sugar and cinnamon in a small bowl; cut in butter until crumbly. Sprinkle evenly over batter.
Bake until a toothpick inserted in the center comes out clean, 22-24 minutes. Cool for a few minutes before removing from pans to wire racks. Serve warm.