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.
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.