Start by making the unicorn's horn and ears – they'll need to dry completely before use. Roll 225g white fondant into a long sausage shape, with one end slightly thicker than the other. Starting from the thicker end, twist the sausage into a tight coil until you have a horn shape. Pinch the small end upwards into a pointed tip. Next, divide the remaining white fondant into two pieces. Roll each one out into a rough oval on a sheet of baking parchment, then pinch the top end of each oval upwards to make a point. Set the ears aside with the horn and leave to dry while you make the cake.
Heat the oven to 180C/160C fan/gas 4. Butter the base of three 20cm sandwich tins and line with baking parchment. Beat the butter, sugar and vanilla together in a large bowl with an electric whisk for 8-10 mins until pale and fluffy. Add the eggs, one at a time, beating well after each addition. Sift in the flour, baking powder and a pinch of salt, and fold in using a large metal spoon. Add just enough of the milk to create a soft dropping consistency.
Divide the batter between the prepared tins and smooth the tops with a spatula. Bake on the middle shelf of the oven for 25-30 mins, or until golden and firm to the touch. Leave to cool slightly in the tins, then transfer to a wire rack to cool completely.
While the cake is cooling, make the buttercream. Beat the butter, icing sugar and vanilla together until pale and fluffy, beating in 4-6 tbsp hot water to loosen the mixture if necessary.
Put one of the cooled sponges on a cake board or stand and spread a little of the buttercream on top using a palette knife. Gently spread over half the salted caramel, if using, then sandwich with another sponge. Repeat the process with a little more buttercream, the remaining salted caramel (if using) and the third sponge. Spread the outside of the cake with a thin layer of buttercream – this doesn't have to be neat. Chill for 30 mins.
Spread a thick, even layer of buttercream over the chilled cake using a palette knife. Divide the remaining buttercream into three bowls and colour each one differently using the gel food colouring. Ours were light pink, purple and dark pink.
Put a little gold lustre dust into a bowl, add a drop of water and stir to make a gold paste. Using a small paintbrush, paint the unicorn horn gold using the paste, as well as the inside of the ears, if you like. Carefully place the gold horn in the middle of the iced cake, with the ears on either side.
Fit three piping bags with three different nozzles (we used a round nozzle and two different-sized star nozzles) and fill each with a different colour of buttercream. Pipe each colour around the ears and horn and all over the top of the cake, coming down one side and over the edge of the front – this is the unicorn's mane. Decorate with some sprinkles, if using.
Divide the black fondant in two pieces, and roll each into a thin sausage shape. Shape each into an arch, then press onto the front of the cake for eyes. If you like, curve the outer edges of the eyes upwards for eyelashes. Cut into slices and serve. Will keep in the fridge for up to three days.