Preheat the oven to 180°C (160°C fan forced). Lightly grease 3 x 18cm round sandwich pans and line the bases with baking paper.
Open the cake box, remove enclosed frosting sachet and reserve for another use. Make up the cake mix as directed on the box using the eggs, butter and milk. Divide the mixture equally between the 3 cake pans and level the tops. Bake for 22-25 minutes or until a round bladed knife inserted into the centre of the cakes comes out clean. Cool a little before turning out onto a wire rack to cool completely. Remove the baking paper.
Sandwich the 3 cakes together with two-thirds of 1 tub of the frosting. Spread the remaining third of the tub of frosting around the top and sides of the cake in a thin layer to seal in the crumbs. Chill in the fridge for 30 minutes.
Spread another two-thirds of frosting from the second tub around the top and sides of the cake and smooth the sides with a cake scraper or large palette knife.
To make the gold cone, trim the top of the waffle ice cream cone to make it level, then spray the cone all over with the edible gold spray or sprinkle over the shimmer powder. Invert the gold cone into the centre of the cake for the unicorn’s horn.
Sprinkle each cut side of the marshmallow with some of the pink sugar sprinkles. Place each, pointed-side up, either side of the horn for the ears.
Divide the remaining frosting evenly between 3 bowls. Using the food colouring pastes or gels, colour 1 bowl of frosting pink, 1 yellow and 1 blue. Drop alternate spoonfuls of the coloured frostings into a disposable piping bag fitted with a medium star nozzle. Pipe swirls and small rosettes of the frosting around the horn and ears and down the back of the cake for the mane of the unicorn.
Sprinkle the rest of the pink sugar sprinkles over the piped frosting. Use the tube of black writing frosting to pipe 2 eyes with eyelashes on the front of the cake. Sprinkle the sugar strands around the base of the cake and use a small palette knife to gently lift and press the strands to the base of the cake. Serve immediately or keep in airtight container for up to 1 week.