Use full-fat cream cheese for the creamiest texture. If using frozen rhubarb or raspberries, thaw and drain them well before mixing with the sugar and cornstarch. Store in an airtight container in the refrigerator for up to 4 days. Freeze chilled squares in a single layer until firm, then wrap and store in a freezer-safe container for up to 2 months. Thaw overnight in the refrigerator.