In a small saucepan over medium-low heat, combine the berries, the lemon juice, 1 tablespoon of the sugar, the vanilla and 1/4 cup of water. Cook, covered, until the berries are softened, about 10 minutes. Stir the sauce, partially mashing the berries. Taste and add the remaining sugar if the mixture is tart.
In a small bowl, stir 1 tablespoon of water into the cornstarch, then stir the mixture into the berry sauce and continue cooking, uncovered, until thickened, 2 to 3 minutes more. Remove from the heat and cover to keep warm.
Preheat a grill or grill pan to medium-low. Brush the pound cake slices with the melted butter. Grill until the cake is toasted, about 2 minutes on each side. Spread the warm sauce on dessert plates, place 2 cake slices on each plate and top with additional sauce.