Place butter in a bowl and use an electric mixer to beat until light and fluffy.
Sift icing mixture into the bowl of butter, then add the salt, vanilla extract.
Now, mix everything together - first use a spoon to mix it all in, and then use your electric mixer to incorporate everything properly (using the spoon first will minimise the amount of icing sugar that fluffs up when you start using the electric mixer).
At this strange the icing will look very dry, to bring it together add a milk, one teaspoon at a time, mixing well using the electric mixer after each addition, until the icing comes to the right consistency. You may need more or less milk then stated in this recipe depending on the temperature of your kitchen and your butter, so be careful to add milk gradually.
If the mixture becomes too runny, you may have added too much milk. You can stiffen the frosting by adding more icing sugar. If you seem to be adding more and more sugar to the frosting, but its just not thickening up enough, its probably because your buttercream is too warm. Stop everything, and put the buttercream in the fridge for 10mins. Then take it out of the fridge and re-whip with your electric mixer. Once you have made your buttercream, prepare your piping bag, by placing a piping tip into the bag (narrow end facing towards the tip) and snip off the end of the piping bag. These are wide piping tips, so snip the bag so that only the very end of the piping tip is exposed at the bottom of the bag.
It's now time to test the consistency of the buttercream. It's easiest to do this step before adding food colour, so you only need to test and adjust one batch of buttercream (instead having to check of 4 or 5 individual batches).To test the consistency, transfer some of the buttercream into the prepared piping bag. Hold the piping bag straight over a sheet of baking paper with the tip positioned about a centimeter above the paper. Squeeze the bag so the frosting comes out and attaches to the baking sheet, then release pressure and lift the piping bag away. You should (hopefully) be left with something that resembles a flower. If not, see the notes below for troubleshooting tips. Once the frosting is at the ideal consistency divide it into 3-5 bowls and use gel food coloring to tint to preferred color. For the green leaves I have colored the buttercream with Matcha powder.
Transfer colored buttercream to piping bags. To achieve a multi colored look spread a thin layer of buttercream around the bag, then add a different color into the middle of the bag.
To pipe onto cupcakes, hold the piping bag so the nozzle is about a quarter of an inch above the cupcake, squeeze the buttercream out, then release the pressure and lift the bag. Start by piping one flower in the center of the cupcake, then pipe a circle of flowers around the center flower.
Fill in any gaps around the flowers using a green buttercream in a piping bag fitted with a leaf nozzle.
Place chocolate bunny ears atop of the buttercream flowers.