Preheat oven to 180C/160C fan forced. Grease a 6cm-deep, 20cm round cake pan. Line base and side with baking paper.
Make cake following packet directions, using the butter, eggs and 3/4 cup water. Spoon into prepared pan. Level top. Bake for 1 hour to 1 hour 5 minutes or until a skewer inserted into centre of cake comes out clean. Stand in pan for 5 minutes. Turn onto a baking paper-lined wire rack to cool completely.
Make milk chocolate cherry fudge. Place milk chocolate, cream and liqueur in a microwave-safe bowl. Microwave on HIGH (100%), stirring every 30 seconds, for 1 to 2 minutes or until melted and smooth. Set aside for 2 hours or until mixture thickens.
Trim top of cake to level. Split cake in half horizontally (each half should be about 2cm thick). Using the base of an 18.5cm round springform pan as a guide, cut a round from each cake half. Discard off-cuts. Grease the springform pan. Line base and side with baking paper, overlapping at join and extending paper 10cm above edge.
Place 1 litre vanilla ice-cream in a bowl. Set aside for 10 minutes to soften (ice-cream should not be melted).
Meanwhile, place 1 cake round in prepared pan to fit tightly, trimming edge, if needed. Stir 1/2 the milk powder into softened ice-cream. Fold in ½ the Cherry Ripe. Spoon mixture over prepared base, using the back of a spoon to create holes in the top of the ice-cream. Spoon 1/2 the fudge and 1/2 the jam into holes and on top of ice-cream. Freeze for 2 hours.
Place remaining cake round on top of fudge layer to fit tightly, trimming edge, if needed. Dollop with remaining fudge and remaining jam. Freeze for 1 hour.
Place remaining ice-cream in a bowl. Set aside for 10 minutes to soften (ice-cream should not be melted). Stir in remaining milk powder. Fold in remaining Cherry Ripe. Spoon over fudge layer, spreading top to level. Cover. Freeze overnight.
Place dark chocolate and cream in a microwave-safe bowl. Microwave on HIGH (100%), stirring every 30 seconds, for 1 minute or until melted and smooth. Set aside for 1 hour or until thick and spreadable.
Stand cake at room temperature for 5 minutes. Using an electric mixer, beat extra cream until just-firm peaks form. Spoon into a piping bag fitted with a 1.4cm fluted nozzle. Remove cake from pan. Transfer to a serving plate. Working quickly, spread top of cake with dark chocolate ganache. Using picture as a guide, pipe cream on top. Decorate with cherries and sprinkles. Serve immediately.