Place your culinary lavender and milk into your small saucepan and heat until bubbles just start to appear on the top.
Set aside and leave to infuse and cool for 20 minutes, then strain using your sieve of tea strainer and discard the lavender.
Using the paddle attachment, beat your softened butter until it is nice and spreadable.
Leaving your mixer running on low, gradually add large spoonfuls of your sifted icing sugar, letting each one mostly beat in before adding another.
Once you have added about half of your icing sugar pour in your lavender flavoured milk and beat in, then continue with your icing sugar until it has all been added.
Ensure the buttercream is fully beaten and no icing sugar remains, then split it equally into 2 bowls.
Add a little purple food colouring to one bowl and mix in thoroughly.
Cut the end off your piping bag and add your piping nozzle. The nozzle should protrude through the end and the bag should fit tightly around it.
Take one of your palette/butter knives and scoop half of your purple buttercream up and add it to one side of the bag. Do the same with half of your uncoloured buttercream on the other side.
Holding the bag vertically above the edge of one of the tiers of your cake (base side up), gently squeeze so as to add a star-shape of buttercream, repeat around the edge of the cake.
Then fill in the centre by just piping in a circular fashion.
Refill your piping back with purple and uncoloured buttercream as per above.
Add the top tier of the cake and ice all over with star shapes, fill in any spaces with smaller piped star shapes so that the cake is completely covered.