Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease a 28cm x 18cm shallow baking tin and line the base and sides with baking paper.
Prepare the brownie mix as directed on the box using the oil, water and egg. Fold the chocolate chunks and chips into the mixture, then spread in the prepared tin.
Bake for 20-25 minutes or until very few crumbs cling to a skewer inserted into the centre. Cool in the tin, then remove the brownie from the tin and peel away the lining paper.
Line the same tin with cling film, allowing it to hang over the two short sides. Place the brownie on a board and carefully cut in half horizontally with a long knife (see Tips).
Return the bottom half of the brownie, cut-side up, to the lined tin. Scoop ice cream evenly over the brownie, spread out and press down. Top with the other half of the brownie, cut-side down. Cover the cake with cling film and freeze for 3-4 hours or until very firm. Freeze overnight for best results.
Remove the tin from the freezer and turn the ice cream-filled brownie slab out onto a chopping board. Cut into 18 pieces. Return to the freezer for 1-2 hours or until very firm again.
Warm the icing to make a glaze following the directions on the tub for 30-40 seconds until runny when stirred. Line a large tray with baking paper. Dip half of each ice cream sandwich in the glaze, allowing the excess to drip off, then place on the tray and sprinkle the tops of the glaze with crushed cookies, nuts or sprinkles (see Tips). Store in the freezer until ready to serve. Can be kept in the freezer for 2-3 weeks.