In a bowl toss together the first three ingredients; Blackberries, sugar and cornstarch, mashing only a few of the berries. Pour this into an 8-inch square cake pan.
Cream together shortening, sugar, and vanilla. Add egg and beat well.
Sift flour, baking powder, and salt and add to mixture, add milk and stir well to mix.
Spoon batter over berry mixture carefully trying to spread evenly.