Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Lightly grease 3 x 18cm round sandwich tins and line the bases with baking paper.
Make up the cake mix as directed on the box using the oil, water and eggs. Divide the mixture equally between the 3 cake tins 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 icing. Spread the remaining third of icing 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 icing 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. 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 glimmer sugar. Place each, pointed-side up, either side of the horn for the ears.
Divide the remaining icing evenly between 3 bowls. Using the food colouring pastes or gels, colour 1 bowl of icing pink, 1 yellow and 1 blue. Drop alternate spoonfuls of the coloured icings into a disposable piping bag fitted with a medium star nozzle. Pipe swirls and small rosettes of the icing around the horn and ears and down the back of the cake.
Sprinkle the rest of the pink glimmer sugar over the piped icing. Use the tube of black writing icing 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.