Lavender chocolate is called cream. I have never seen one in person, but from pictures they appear to be a cream colored almost off white. Since lavender is recessive your best bet is to cross your lavender drake with chocolate females. This will give you black males split to chocolate and lavender and black females split to lavender. Take one of your split drakes from this breeding and cross him with his sisters. It may take a while but you should be able to hatch some cream ducklings. However any creams that hatch from this cross will be female since it is a sex linked gene and the females only need one copy to show the color.
Hope this makes sense.