1Preheat oven to 325^. Melt 1 stick of butter in a 9x13 pan in oven. Remove from oven; sprinkle brown sugar evenly over butter. Spoon blackberry filling evenly over brown sugar; saving some for topping. Set aside.
2Beat remaining butter at low speed with electric mixer until creamy. Gradually add powdered sugar, beating until blended. Increase mixer speed to medium; beat mixture 2-3 minutes or until light & fluffy, stopping to scrape bowl as needed.
3Add eggs one at a time, beating at low speed, just until blended. Add vanilla. Gradually add flour and water; beat 1 minute. Scrape bowl; increase mixer speed to medium and beat 2 minutes. Do not overbeat. Batter will be light and smooth.
4Spoon batter over blackberry filling; spread gently to cover. Bake 55-60 minutes or until a wooden pick inserted in center comes out clean. Remove to wire rack; cool completely.
Cut into 12 squares. Lift each square and invert onto serving plate. Top each serving with remaining fruit filling. Microwave 10 seconds on high and serve with ice cream, if desired.