Place 1/4 cup water in a small microwavable bowl. Sprinkle the unflavored gelatin on top; let stand 5 minutes. Microwave until gelatin dissolves, about 25 seconds. Stir in sour cream until smooth. Pour the mixture into a zip-top back; press out excess air and seal. Let cool, kneading bag occasionally, until thickened, about 15 minutes.