Combine all ingredients in a food processor and process. You can roll it or put it into a container at this point. It should be chilled in the refrigerator overnight. If you are going to slice it, do not bring it to room temperature, but if you are going to serve it from a container, then leave it out for a while before serving.
The longer the mixture sits, the more flavorful it becomes. Use only sweet butter, as using salted butter would make the product too salty. This is beautifully served with Melba Toasts.