Stir in the tarragon. Scrape into a small bowl and cover. Or, make a log of butter by spooning it onto a piece of waxed paper, folding paper over the butter and rolling it gently until you have a sausage shape, then twist the ends to seal. Refrigerate for at least 2 hours and up to 4 days. You can also wrap the log in foil and freeze for up to 3 months; thaw it in the refrigerator.