Mix 1 package of yellow cake mix by directions. Steam 3 pints of fresh blackberries in 2 cups water for 3 minutes. Pour blackberries in 10 x 13 oven proof serving pan. Add 1/2 cup butter, 2 cups sugar and 1 teaspoon vanilla; mix into berries. Spoon cake mix evenly over entire pan. Bake at 350 degrees about 35 minutes on lower rack until brown. Test with knife to check if baked through. Serve 9 to 10.