In a large bowl beat the butter and brown sugar till creamy. Add the egg and vanilla and beat till smooth.

In a separate bowl, sift the flour, salt, and baking soda, set aside. In another bowl mix lemon juice, lemon rind, milk and sour cream together. Add the flour mixture a little at a time to the butter and brown sugar mixture, alternating with the milk mixture. Beat till well blended and creamy smooth.

Fold in rhubarb and nuts.

If using the super size paper baking cups set them in a small dessert bowl, and place bowls on a large baking sheet.