My question is -- does this exist? I have a contact In Yangon who has asked if I want this bus. Yes, I do. but I can find no reference to it anywhere. Can you help?

I doubt you'll be able to cover that distance in one go by road. Kalaw is in the hills on the way to Inle Lake, and quite slow going by road. It's a long way from Yangon.

If you don't get any more info, then have a look on the Myanmar forum on Trip Advisor.

There certainly are daytime buses, but LL is correct, that is a very long journey and won't be accomplished in one day. I've not taken public buses in Myanmar, so I am not there best source of info on the buses. Personally, I find Trip Advertiser worthless. There are a number of very knowledgeable people at Thorntree, the Lonely Planet travel forum.

I would suggest that if you want to do this bus journey, you break it up. Perhaps Yangon to Bagan, then to either Mandalay or Inle Lake, then to Kalaw.

"but I can find no reference to it anywhere" - there is much in Burma that is like this. Things that don't exist on the internet do exist in Burma.

The TA discussion forum for Myanmar has an excellent DE called Silverswimmer, who puts a lot of time and effort into keeping the info on there as up to date as possible.

Silverswimmer is a regular on Thorntree and posts here from time to time as well. He does have excellent knowledge of Myanmar.