In a small saucepan, place the frozen berries, sugar, vanilla, and 1 cup of water. Heat over medium-high heat until mixture comes to a low boil, and berries begin to soften and come unstuck.
In a bowl, whisk together the cornstarch and ¼ cup water, and add to the saucepan while the saucepan is on the stove.
Turn the heat down, and let the mixture simmer until it is thickened to desired consistency.
Serve hot or cold, over ice cream, cakes, muffins, or biscuits.