Preheat oven to 180°C (350°F).
In a large bowl, fold the peaches, sugar, cinnamon, lemon juice, vanilla and cornstarch until all ingredients are well combined.
Pour butter into 26x10 cm (10x8 inch) baking pan.
In a large bowl mix flour, baking powder, sugar and milk until all of the ingredients are incorporated. Pour batter over butter (do not stir).
Layer the peaches and blueberries on the top (do not mix).
Bake around 40 minutes (depending on your oven), or until golden brown.