Cut the stems off the sorrel, shred the leaves coarsely, and wash. Dry carefully and completely.
Take a non-reactive fry pan, and melt a teaspoon of butter in it. (Add more, if needed to prevent sticking.) Add the sorrel leaves, and stir as they wilt. They will cook down very quickly. Allow to cool.
Puree the sorrel and the remaining butter in a food processor until smooth.
Refrigerate until use.
Notes
Be sure to use a non-reactive pan - sorrel is high in oxalic acid, and will react with aluminum or cast iron.