I’ve seen claims that, while you can replace heavy cream with the proper proportions of milk and butter in recipes that don’t require whipping, the mixture will not form stiff peaks when whipped, even if it contains the same milkfat percentage as heavy whipping cream (>35%). However, offhand I don’t know why this would be so, and I can’t find an explanation of the claim.
I’ve used milk and butter to stand in perfectly well for cream in things like sauces or drinks, but I’ve never tried whipping it. If it’s not possible, why? Does it have something to do with the proportion of protein, fat, and water? Does the process of churning butter break down protein/fat structures that are required for holding peaks? Or is it not impossible, but simply difficult or time-consuming to get the butter to emulsify properly back into the milk and cool to the point that it can hold aeration?