Stir the sugar and cornstarch mixture into the berries.
Let the mixture rest for at least five minutes while you prepare the dough. This will let the sugar draw out the juices of the berries, a process called maceration.
For the batter
In a separate bowl, sift together the flour, baking powder, and salt
Add in the milk and stir with a wooden spoon or wire whisk until mixed.
Add the melted butter and vanilla and mix until combined
Put it together and bake
Prepare an 8 inch square cake pan with non-stick spray.
Pour the batter into the pan
Top the batter with the berry mixture. Be sure to scrape all the juices and sugar onto the batter as well.
Bake for 50 minutes or until golden brown (It took only 43 minutes in my oven)