Grease a 22cm springform pan. Line base and side with baking paper. Place sugar, blackberries and 2 tbs water in a small saucepan over medium heat. Cook, stirring occasionally, for 3 minutes or until berries are glossy and sugar has dissolved. Set aside to cool completely.
Meanwhile, place cookies in a food processor and blitz until fine crumbs form. Place crumbs and hard set chocolate in a medium bowl and mix until combined. Place into base of pan and, using back of a spoon, press into an even layer. Freeze for 1 hour or until firm.
Spoon vanilla ice-cream over cookie base and, using back of a spoon, level surface. Freeze for 1 hour or until slightly firm. Scatter over two-thirds of the drained blackberries and freeze for 2 hours or until firm.
Spoon blackberry ice-cream over blackberries and, using back of a spoon, level surface. Freeze for 4 hours or until firm. Transfer cake to a plate or board. Serve topped with remaining drained blackberries.
