In a saucepan over medium heat add 1 1/2 cups raspberries, sugar, and water. Stir to dissolve the sugar. Heat until the mixture comes to a boil, stirring frequently.
Reduce heat and add the vanilla.
In a small bowl stir the cornstarch and 2 tablespoons of water.
Add the cornstarch mixture to the saucepan of raspberries. Stir and boil over medium-low heat for 4-5 minutes or until the sauce has thickened a bit.
Remove the pan from the heat. Add the butter and stir the butter into the sauce until the butter is completely melted.
Let the raspberry sauce cool for 20 minutes, then add the remaining raspberries (about 1-1 1/2 cups or whatever you have left. Stir to incorporate the raspberries into the sauce.
Serve over any and all desserts!
Store in an airtight container in the refrigerator.