Bake the cake as directed on the box
In a medium sauce pot combine the blackberries, sugar and cornstarch and bring to a boil. You can mash the berries with a spoon if you prefer not to have bigger berries in the filling. I smashed them up a little bit. You want the mixture to be a little runny so it soaks into the cake. Let it completely cool.
Once the cake is out of the oven let it cool for about 15 mintues and poke holes all over the cake. I used a straw.
Pour the blackberry mixture all over the cake once the mixture is cooled.
Combine heavy cream, powdered sugar and vanilla into a mixing bowl with the whisk attachment. (Tip: I stick my bowl and whisk into the freezer for about 10 minutes to make sure it's very cold)