In a saucepan over medium heat, stir 1.5 cups rasberries, sugar, and water until the sugar is fully disolved.
Heat mixture to the boiling point stirring often. Reduce heat and add vanilla.
In separate small bowl, combine cornstarch and 2 Tbls water. Pour into rasberries mixture. Stir and bring to boil. Simmer and allow to thicken as desired.
Remove from heat and add butter to the mix, stir until melted.