Mix the Golden Syrup and butter till smooth
Gradually add the icing sugar, until the right consistency is achieved.
Add colouring, and kneed well.
The result is NOT white, it's yellow, so you'll need to colour it.
A 10g green gel created a nice pastel green.
We made a second, half size batch, with 90g of butter and 90g Syrup, and this nicely covered a 9" square cake, with enough left over for decoration.