Q:

Could I add butter to single cream to make whipped/double cream?

I want to make some whipped cream for millefeuille but I only have single.
If I added butter to single cream and whipped it, would that add enough butterfat to it to make it suitable for whipping?
Thanks in advance. 

A:

Yes, this is possible but you need a high-speed blender like a Thermomix or Vitamix to do so. There's actually a recipe on the Thermomix website:
http://www.ukthermomix.com/recshow.php?rec_id=29

Ingredients
250 gm unsalted butter
250 gm milk (full cream or semi-skimmed)
Method
Weigh butter in pieces and milk into the Thermomix bowl.  Cook 3 minutes/ 90C/ Speed 1.  Blend 30 seconds/ Speed 8.  This emulsifies the butterfat back into the warmed milk and makes a perfectly delicious cream.  Chill in the fridge 4 hours to use as pouring or spooning cream.  Chill in the fridge 8 hours or more to be able to whip the cream.

I presume you could tweak the ratio to account for the difference in fat between milk and cream to make your double cream.

A:

No, it won't work. 
Michael's comment explains why. Whipping cream is not just fat and water mixed, it is fat and water emulsified. This is a big difference. 
If you had some special reason to do this on a regular basis, you could get it to work by adding emulsifiers. You can beat any fat with water and lecithine or xanthan and get a creamy result. As far as I know, this is how plant based cream substitutes work. 
But if the issue is that you just don't have whipping cream right now at home, then it is easier to go buy whipping cream than to go find emulsifiers (I buy mine online, don't know if there are brick-and-mortar B2C stores which sell them). If for some reason you absolutely can't do it in time, I would suggest using a different filling. Pastry cream works well with millefeulle, buttercream can work too, lightened with whipped eggwhites if necessary. 

