What I do in order to make pistachio butter (I like making homemade pistachio ice cream) is to grind the shelled/skinned pistachios into a fine, flour-like consistency either in my coffee grinder or via my hand-held, manual rotary cheese grinder that has fine grater attachment and does a wonderful job of making fluffy nut flours. Although it's harder work, I'm partial to my rotary grinder because there's less heat involved. From there, you can either add enough bottled pistachio oil (or else some other neutral tasting oil) to make plain pistachio butter with a nice spreadable consistency, or if you are making sweet treats like ice cream with it, you can forego the oil and add a little sweetened condensed milk instead.
IrishLass