In a saucepan, combine the raspberries, 3/4 cup of water and the sugar. Bring to boil. Simmer for 15 to 20 minutes or until the raspberries have broken down. Remove the mixture from the heat and strain in a fine-mesh sieve. Return the strained mixture to the heat. Dissolve the cornstarch in the remaining 1/4 cup of water. Whisk the slurry into the raspberry mixture. Bring the mixture back to a boil and simmer for 5 more minutes. Remove from heat and cool completely.