1. Preheat oven to 375˚F (190˚C). Grease a 9 x 13” baking dish. Toss together berries, sugar, cornstarch, lemon zest, lemon juice and butter; scrape into prepared pan.
2. Topping: Stir together flour, sugar, baking powder and salt; using a pastry blender or fingertips, cut in butter until mixture resembles coarse crumbs with a few larger pieces. Whisk milk with vanilla; gently stir into flour mixture using a fork to make ragged dough. Drop heaping tablespoonfuls randomly over filling. Brush tops lightly with additional milk.
3. Bake for 45 to 50 minutes or until filling is bubbling and biscuits are golden brown and cooked through.
4. Serve warm with ice cream.