Pink, orange, yellow, green, blue and purple food colouring
2 cups unsalted butter, room temperature
1 teaspoon vanilla extract
5 cups confectioner's sugar
Pink, yellow, green, blue and black food colouring
24 mini marshmallows
Pink sanding sugar
Bake the cupcakes: Combine the flour, salt and baking powder in a bowl. In a separate bowl, cream the butter with an electric mixer until pale and fluffy. Add the sugar and mix. Add the vanilla and eggs and beat until combined. Add the dry ingredients in 2 additions, alternating with the milk. Divide the batter into 6 bowls and dye them the colours of the rainbow. Dollop a small spoonful of each colour of batter into lined muffin tins, until there is a dollop of each colour in each muffin tin. Bake the muffins at 350F for 15-20 minutes, until fully cooked. Allow them to cool completely.
Buttercream: Cream the butter with an electric mixer until pale and fluffy. Add the vanilla extract and combine. Add the confectioner's sugar one cup at a time, then beat for 3-5 minutes until fluffy.
To decorate: Evenly coat the surface of the cupcakes with the white buttercream. Divide the remaining buttercream into 6 bowls. Keep one bowl white and dye the remaining bowls pink, yellow, green, blue and black. Place a piping bag fitted with a star-shaped piping tip into a tall glass. Spread 4 vertical stripes of white buttercream onto the insides of the piping bag, evenly spacing them apart. Spread the pink buttercream into one open space and repeat with the yellow, green and blue buttercream. Squeeze enough buttercream out of the tip until all 5 colours come out. Place the black buttercream into a piping bag fitted with a round piping tip. Pipe swirls of buttercream to create the unicorns' mane, with a large dollop in the center where the horn will go. Cut the mini marshmallows in half diagonally, to create ears. Dip the cut, sticky side of the marshmallows into pink sugar to create the pinks of the ears. Stick the ears onto the cupcakes, then stick the candle horns into the cupcakes. Pipe the eyes and eyelashes onto the cupcakes and enjoy!