Cream together the butter and sugar until light and fluffy.
Add the two eggs and beat until just combined.
Sift in the flour and bicarbonate of soda and beat again until just combined.
Divide the batter into three bowls. Using a cocktail stick, add a drop or two of food colouring gel to each bowl. Fold to combine the colour.
Use a teaspoon to add some batter from each of the bowls to the cupcake cases, filling to two thirds full.
Bake in the oven for 20-22 minutes.
While the cakes are baking, make the buttercream. Beat the butter using an electric whisk or mixer on high for 5 minutes, until pale, light and fluffy.
Add the icing sugar in stages, whisking slowly to combine at first and increasing the speed as the icing sugar is combined.
Add 1-2tbsps of milk and 1tsp vanilla extract and whisk until soft.
Remove the cakes from the oven and leave for at least 30 minutes to cool.
Using a piping bag and star-shaped nozzle, pipe the buttercream icing onto the cupcake, starting in the middle and working your way out to the edge.
While the buttercream is still soft, cover in multi-coloured sprinkles and edible glitter! You can make a ‘unicorn horn’ by wrapping fondant around a cocktail stick and inserting into the centre of the cake.