In a saucepan over medium heat, stir together 1 ½ cups raspberries, sugar and water until the sugar is dissolved. Heat the mixture to boiling, stirring often.
In a small cup, combine the cornstarch and two tablespoons of water. Mix well. Pour this into the boiling raspberry mixture. Stir and boil over medium-low heat for 4 minutes or until the mixture has thickened.
Remove the pan from the heat. Add 1 tablespoon of butter and mix until the butter has completely melted.
Allow the mixture to cool for 15 minutes, then add the last of the raspberries and stir to mix.
Serve over pancakes, cheesecake or angel food cake!