In a saucepan over medium heat, combine the berries, 1/2 cup of water, sugar and lemon juice. Stir frequently, and bring to a simmer.
In a small bowl, mix the cornstarch with 2 tablespoons of cold water. Slowly stir the corn starch into the berries, taking care not to crush the berries but being sure to whisk out any clumps.
Simmer until the sauce is thick enough to coat the back of a spoon, about 5 minutes.
Remove from heat and gently stir in vanilla and lemon zest.
