Cake: Preheat oven to 170 degrees C. Grease and line bottom of 24cm springform cake tin.
Add the grated zest to the cake mix and make the cake according to packet directions then pour into prepared tin.
Cook for 40 to 45 minutes or until a skewer comes out clean. Cool in the tin for 15 minutes then remove to wire rack, removing the sides and bottom. Cool completely then store in airtight container until ready to use.
Ice Cream: Line the 24 cm spring form tin with plastic wrap, leaving some overhanging the sides for easy removal of ice cream.
Using an electric mixer, in a large bowl, beat the condensed milk, cream, grated zest and food colouring together until thick. Stir in choc bits.
Pour into the prepared tin then level the top, cover with plastic wrap and freeze for at least 8 hours.
Ganache: In a heatproof bowl over a saucepan of boiling water, melt the chocolate with the cream, stirring until smooth. Remove from heat and cool, stirring occasionally. Allow to thicken slightly.
To Assemble: When the ganache is ready, cut cake in half width ways, (side to side) and place bottom of cake on a plate or cake board.
Remove the ice cream from tin and plastic wrap then place onto the cake bottom. Cover ice cream with top of cake.
Spread the ganache across the top of the cake, letting some run down the sides of the cake as well. Decorate with the chocolate and orange slices.