Fill a pastry bag fitted with a small writing tip (or a small plastic food-storage bag with a small corner snipped off) with the remaining 1/4 cup sour cream. Pipe the sour cream, beginning in the center of the pie, in continuous spiral to the outer edge of pie, with about 3/4- to 1-inch space in between each spiral. Lightly draw a long wooden skewer or the dull back of a knife from the center of the pie through the spirals to the outer edge of the pie. Then make another line, drawing the skewer from the outer edge back to the center. Repeat drawing the lines, back and forth, around the entire surface of the pie, spacing as desired. Refrigerate for 1 hour to firm.