Berries: Toss together blueberries, raspberries, strawberries, sugar and cornstarch in large bowl until well mixed Spoon berry mixture into ungreased 8*8*2-inch square baking pan, spreading mixture evenly.
Topping: In same large bowl, beat together sugar and butter on medium speed until light and fluffy, about 3 minutes.
Beat in egg and vanilla until well blended Mix together the flour, cornmeal, baking powder and salt in a small bowl.
Add half the flour mixture to the butter mixture.
On low speed, beat just until combined.
Beat in the milk, then the remaining flour mixture.
Drop the dough by large spoonfuls over the berry mixture in the baking dish Bake in 350°F oven for 1 hour and 20 minutes or until top is golden and fruit is bubbly.