Step 3
In a large bowl with an electric mixer on medium-high speed, beat butter until creamy, about 2 minutes. Add sugar and brown sugar gradually, beating until light and fluffy, about 3 minutes, scraping down the bowl once or twice. Beat in vanilla, then beat in eggs one at a time, allowing each one to be absorbed before adding the next. Add about one-third of flour mixture and mix on low speed. Gradually add remaining flour mixture, mixing just until blended. Batter will be thick. Chop one 6-ounce package of blackberries finely and fold into batter, along with chopped chocolate. Spread evenly into prepared pan.