Preheat the oven to 180ºC/fan 160ºC/gas mark 4. Line a 12-hole muffin tin with cases. Beat together 120g butter with the caster sugar until light and fluffy. Add in the eggs, vanilla and flour and beat until combined. Spoon into the paper cases and bake for 15-20 minutes or until a skewer inserted into the centre comes out clean. Cool completely on a wire rack.
Meanwhile, make the icing. Using an electric whisk, beat the remaining butter in a large bowl until fluffy. Add half the icing sugar and continue to beat until smooth. Add the remaining icing sugar and continue beating until creamy.
Divide the icing between three bowls and colour each differently for a pastel rainbow of cupcakes. A little food colouring goes a long way, so start with a drop or two and mix well.
Fit a large nozzle on to a piping bag and fill with icing. Hold the nozzle end with one hand and squeeze the top of the bag with your other hand to make the icing flow through. Start by making a circle around the edge of the cakes and work towards the middle. Twist the bag to finish. Decorate with the mini eggs.