Preheat oven to 375˚F (190˚C). Grease 8-inch (2 L) square baking dish. Toss together berries, sugar, cornstarch, lemon zest, lemon juice and butter; scrape into prepared pan.
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.
Bake for 45 to 50 minutes or until filling is bubbling and biscuits are golden brown and cooked through.
Serve with vanilla Greek yogurt or ice cream.
Substitute thawed and drained mixed frozen berries for fresh berries if desired.
This classic, one-pan meal is perfect for those nights when you want to treat your family to something delicious and hearty without the big cleanup. For added creaminess, add a generous dollop of sour cream!