In the bowl of a stand mixer (or using hand held beaters) place your cubed butter. Turn it up to high speed and whip the butter until it is very light in colour and beautifully creamy. Ideally about 5 minutes.
Add 2 cups of Icing Sugar and turn your stand mixer back on, only a low setting to start. Once the sugar has started to incorporate turn it back up to high for a minute. Add in the vanilla extract and whip again for 30 seconds.
Add your remaining 2 cups of icing sugar and follow the same process. Start on low until well mixed, then up to high speed again for 3 minutes.
Add in the milk and slowly mix together. Then pop back onto a high setting and beat for another 3 minutes.
Now it's time to add your colour! If you'd like to do an ombre` effect like I have here simply add a small amount of dye to start with. Once happy with your first (lightest) colour scoop a good 2-3 heaped tablespoons of frosting out into a piping bag and set aside.
Pop a few more drops of the same colour into the frosting again, (repeat the step above) mix to combine. Once happy with the colour scoop a few tablespoons out into another piping bag and set aside.
Finish with a larger quantity of frosting which will not only be your top ring colour, but the top and middle of the cake. Add a few more drops of dye and beat. Transfer a few tablespoons to a final piping bag and set aside.
To assemble the cake place a little dob of frosting in the middle of your serving plate and place your first cake upside down on top of it. This will act as glue and hold your cake in place!
Dollop on and smooth out frosting (as much or as little as you like). This will sandwich between your two layers of cake. Place the second cake on top right away,
To create the ombre` effect: Pipe one solid line of your first icing colour around the base of the cakes. Follow this with a second line in your second colour and finish with your third.
Using an offset spatula or straight knife (being sure to keep it the same way up the entire time) start scraping the frosting lines in a continuous motion. Scrape off any extra as you go and back into the bowl. This will blur your lines of colour and give you that gorgeous ombre` look.
Finish off the cake by piling the remaining frosting on top and smoothing off!