1Drain raspberries, reserving 1/3 cup syrup. In 1-quart saucepan, mix sugar and cornstarch. Stir in reserved raspberry syrup. Cook and stir over medium-low heat until mixture thickens and boils. Boil and stir 1 minute longer.
2Stir raspberries into sugar mixture. Cool completely, about 30 minutes.
3Leftover filling can be tightly covered and refrigerated up to 5 days; do not freeze. Let stand 30 minutes at room temperature to soften; stir before using.