That's butterfat on top. I used mostly half and half with about a cup of added whipping cream, and a cup of powdered milk. It made really creamy and thick yogurt. Not very tart. I've got about eight or ten cups of it in the fridge, chilling. I mixed half a cup of sugar and a lot of vanilla extract in, and I'll try the kids on the (probably) 15% yogurt when everyone gets up. This might be a bit of a hassle, making it, but it's $1.99 per litre of half and half, versus $10 for an equivalent amount of high fat yogurt from the store. My children can eat that much in one sitting.
Next time I'm making more.