With a pastry brush, coat the bottom of a 13 x 9 or large oval dish with melted butter. Evenly spread mixed berries on the bottom. Place dish on a lined baking sheet.
In a medium bowl, mix butter, sugar, flour, milk, baking powder, salt and vanilla with a wooden spoon. Spoon batter over fruit and spread evenly.
Mix remaining 1/4 cup sugar and 1/2 cup warm water. Pour evenly over dish. (This creates a golden brown crust and makes the fruit more soft and sweet)
Bake for 50-55 minutes, until topping is golden brown and fruit is bubbling.
Serve warm as is, or with vanilla whipped cream (recipe below) or vanilla ice cream.