Preheat oven to 350F. In a medium bowl mix your blackberries with the brown sugar and vanilla, stir to blend and pour in bottom of 2qt. casserole dish that has been sprayed with non stick spray.
In another bowl mix your sugar cookie mix with your soft stick of butter and the egg until soft dough forms. With clean finger or a cookie scoop place cookie dough all around the top of the berry mixture until cookie dough is used up and some berries are showing.
Place casserole dish on a cookie sheet and place in preheated oven. Bake for 20-25 minutes or until cookie topping is golden brown. Serve with a scoop of ice cream if desired.