Step 1
Divide the ready-to-roll icing into 12 portions. Roll out each portion on a clean work surface lightly dusted with icing sugar to a 2-3mm-thick, 10cm-long log. Fold 1 piece in half and gently twist to make a horn shape, gently rolling 1 end between your fingertips to make a point. Repeat with the remaining pieces to make 12 horns. Place on a baking tray lined with baking paper. Set aside at room temperature overnight to harden.
Step 2
Preheat oven to 180°C. Line a 12-hole, 1/3-cup (80ml) muffin pan with paper cases. Whisk the flour, baking powder and bicarbonate of soda in a bowl.
Step 3
Use an electric mixer to beat the butter in a bowl on high speed for 1 min or until pale. Add the sugar and beat, scraping down the side of the bowl occasionally, for 2 mins or until smooth and creamy. Add the egg whites and vanilla. Beat on medium-high speed until combined. Beat in the sour cream, scraping down the side of the bowl occasionally.
Step 4
Add the flour mixture to the butter mixture and beat on low speed until just combined. With the motor running, gradually beat in the milk until combined (don’t overmix).
Step 5
Divide evenly among paper cases. Bake for 20-25 mins or until a skewer inserted in the centres comes out clean. Transfer the cupcakes to a wire rack to cool completely.
Step 6
Meanwhile, to make the unicorn buttercream, use a clean electric mixer to beat the butter in a bowl until pale. Add the icing sugar, in 3 batches, beating after each addition until combined. Add the cream and vanilla and beat until combined.
Step 7
Divide the buttercream evenly into 4 portions. Tint each portion with red, blue, green or yellow food colouring. Place a piece of plastic wrap on a work surface. Arrange the buttercream portions side-by-side on the wrap. Use your hands to roll the plastic wrap into a log and twist the ends. Cut off 1 end and place in a piping bag fitted with a fluted nozzle. Pipe buttercream over the cupcakes. Decorate with the unicorn horns and confetti.