Is there any way to make single cream to double cream?
Adding butter to cream will not work because you're not actually increasing the fat content of the cream-portion of your mixture. Cream is made up of microscope globules of butterfat surrounded by phospholipids (membranes of fatty acids that act as emulsifiers). This is what prevents the fat in the milk from congealing into a fatty mass (i.e. butter). The butter-making process breaks down these membranes and allows the butter to form. Butter is also a water-in-oil emulsion, but you can't re-emulsify it into cream by simply adding it back to a lower-fat cream base.

An experimental solution

(This is highly experimental, so I'll remove it if someone says it doesn't work.)

If you can only get 25% homogenized cream, try freezing and then thawing it. This should undo the homogenization to some degree, so it should separate when defrosted. Let the fat rise to the top, and pour off the excess cream (or drain off the whey from the bottom) to get the total milkfat content up into the "whipping cream" range.

(This may not work. My concern is that this separate-by-freezing process won't actually give you whey + cream&hellip; for the same reason cream + butter won't work. The emulsion may be destroyed by the separate-by-freezing process. It may indeed create a higher-fat solution&hellip; but the resulting concoction may not be "whippable". Try it and report back. Thanks.)