Hi,
Regular butter icing would be the way to go for piping. You can colour it, flavour it etc. I use 2oz butter to 4-5oz icing sugar, and put a drop of whatever colour you want into it and mix really really really well - the more you mix it the lighter it is. I bought a packet of Dr. Oeatkar tubes of coloured stuff for writing.
Best of luck with the baking - put up a pic and show us when you've it done.
I just googled 'aster' and found this is another flower that grows/blooms in a zillion different versions
Exactly which version are you wanting? Then I can help you
Post a pic of what you are looking for. Some of what I saw is basically nothing more than a daisy; others are multi layers
of short rounded petals.
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