Try BM "Standish White". It's a creamy yellow, soft and not overpowering. Also, BM "Windham Cream" is a nice creamy color suitable for any room.

Hi Tracey, do you have pictures of your current bathroom? is this the sort of color you want to achieve?

For a buttery yellow, as I interpret "buttery" to be, the tiniting needs have have more than just yellow. A dash of red tint brings in a nearly imperceptible amount of orange to give the yellow that buttery complexity. SW 6896 Sole does just that.
But it too will look garish when paired with your glass tile, gray cabinets and slate floor. So, you're on the right track to go for a creamy or ivory type yellow. A good one is SW 7683 Buff.
I hope this helps.

Thanks, all good suggestions! I picked up some BM Standish White today and will also test SW7683.

We painted my mother's kitchen with Restoration Hardware's Butter - it was so inviting. Their entire Butter-family is pleasing - maybe their butter cream is close to what you are looking for? http://www.restorationhardware.com/catalog/product/product.jsp?productId=prod1161244&categoryId=cat1017001