In small saucepan combine berries and syrup with cornstarch. Cook and stir until thickened and bubbly. Reduce heat; cook and stir 2 minutes more. Remove from heat. Immediately press through sieve; discard seeds. Transfer to bowl. Cover surface of filling with plastic wrap. Cool without stirring. Chill at least 2 hours; for maximum stiffness, chill overnight. Makes 2/3 cup.