1. For the cake: Preheat an oven to 180 degrees (fan), or equivalent. Line the base and sides of a high-sided 20cm loose-base cake tin with parchment paper.

2. Using a stand mixer, cream the butter and sugar until they are pale and fluffy. Gradually add the eggs. Switch to a low speed and add the self-raising flour, vanilla and orange zest. Spoon the batter into the lined cake tin.

3. Bake for 70 minutes in the preheated oven, until the cake has risen and is golden and a skewer inserted into the centre comes out clean. Leave in the tin to cool for 30 minutes, then transfer to a wire rack to cool completely before icing.

4. For the buttercream: Add the sieved icing sugar into the butter in two batches and beat until combined. Beat in just enough orange juice to give a smooth, spreadable consistency. Divide the buttercream between three or four bowls and make up your desired colours (use the tip of a cocktail stick dipped into coloured pastes to add the colour).

5. Slice the cake into three even layers. Spread a thin layer of buttercream (take equal amounts from the coloured icings), then jam, over two slices and layer one on top of the other, with a buttercream and jam layer on top.

6. Slice away the bump from the top layer of cake to give a flat surface. Cover with a thin layer of buttercream and place over the top of the cake.

7. Transfer the cake to a turntable (if using) or flat plate. Use a palette knife to add wide bands of coloured buttercream to the cake sides (avoid drawing the loose cake crumb into the icing). Refrigerate to harden the buttercream before repeating with a second layer (slightly thicker). Scrape the long edge of the palette knife around the outside of the cake to give a smooth, colour-blended finish. Refrigerate.

8. For the white chocolate drip: Heat the white chocolate until just melted. Pour a thin ribbon of warm chocolate along the rim of the chilled cake, using a teaspoon to drip it over the edge at intervals.

9. Chill all the remaining buttercreams, and when they are very cold, form into separate coloured sausages on a sheet of clingfilm. Place alongside each other and use the clingfilm to press them into a single, thick sausage. Remove the clingfilm and transfer the sausage inside a piping bag fitted with a medium-sized star nozzle. Pipe a mane of rosettes on to the front of the cake. Once piped, keep the cake refrigerated.

10. For the unicorn horn and ears: To form the horn, roll out a quite thick, tapered rope of white fondant icing and twist it around a wooden skewer until you reach the top. Check the photograph for the effect you are looking for. Cut out two ear shapes. Paint the horn and ears with edible gold colouring and place on top of the cake, along with any extra sprinkles.

Variation: Instead of one large tin, divide the cake mixture between three lined 20cm sandwich tins and bake for a reduced time.