Heat the butter and bittersweet chocolate in a double boiler, stirring frequently, until the butter and chocolate are melted. Set aside to cool. Meanwhile, blend the eggs and the sugar until the mixture is thick and frothy. Gradually fold the egg mixture into the chocolate mixture, being careful not to over-stir. In a separate bowl, combine the flour, cocoa powder, and salt. Fold the dry ingredients in with the chocolate-egg mixture, again being careful not to over-stir. Pour batter into a greased 9"x12" pan. Top with blackberries. Bake at 350 degrees for approximately 25 minutes, or until top is crackled and begins to feel firm, and edges pull away from the sides. (Check first at 20 minutes so as not to overbake.)