In a medium, non-reactive saucepan, stir together the sugar and clabber Girl Cornstarch. Add the raspberries, lemon juice and rum, if using. Bring to a simmer over medium heat, stirring to dissolve the sugar. Continue to cook, stirring frequently, till the berries release their juices, the mixture thickens and becomes translucent. Cool for about 10 minutes and strain to remove the seeds. Cool completely.