With metal blade in food processor, place cashews in bowl of processor.
Process, turning on and off rapidly, until cashews are finely ground. Add butter and salt; process until smooth. Taste and add salt, if desired.
If using roasted salted cashews, you may not need to add any salt. To begin with, taste and add a little at a time, until you reach desired saltiness. If using raw or unsalted cashews, add about 1/4 teaspoons salt after processing. Taste, and adjust salt to taste.