Preheat oven to 375°F and butter an 8x8-inch baking dish.Toss berries with 1/4 cup sugar, cornstarch, and lemon juice, then spread in dish.Whisk flour, remaining sugar, baking powder, and salt in a bowl.Stir in melted butter, milk, and vanilla until smooth.Pour batter over berries without stirring.Bake 40 to 45 minutes until golden and bubbly.