Yogurt made with heavy cream

I am skeptical that butter from yogurt is a thing.

When yogurt is made the milk proteins denature and form a mesh that traps all the large molecules in the milk. Water, sugar, and some small molecules can come out but the fat never does- it's huge and tightly bound up in the gel.

Even when yogurt is blended up the whey will separate out but the fat never does. I have a hard time believe that extricating the fat from that protein mesh will be easier than just letting it rise to the surface in milk.

Perhaps thoroughly cooking the yogurt would melt out the fat but you would still lose the rest of the milk solids and would have ghee.

With purchasable yogurt it would be even less feasible because it is often made from low-fat milk to begin with.

EDIT-

Searching online I was able to find Indian recipes for making butter from yogurt: http://www.youtube.com/watch?v=_vK8hW_oSu0
In this case they made yogurt from heavy cream. The goal was a cultured butter- not easier butter.

You cannot make double cream by mixing milk and butter, this is wishful thinking. Some recipes which call for heavy cream, e.g. a cream soup, can indeed be made by adding milk and butter in such quantities that the fat ratio becomes similar to double cream, and will be successful. But for all such cases, there is no reason to mix the milk and butter before adding them to the other ingredients, you can step the step which supposedly makes cream.

A mixture of butter and milk will always be unstable, no matter the temperature at which you added them. You could in theory avoid it by using an emulsifier and a very thorough mixing method (high powered blenders are good here), but the result will have a different taste and somewhat different texture from cream and still won't whip. I've never seen a use case where doing it makes any sense.