Blend the sour cream and heavy cream together in a bowl using a wire whisk. Cover with a wet towel and let stand on the kitchen counter for 6-8 hours. Refrigerate. When ready to serve, whisk in in the powdered sugar. Use more or less powdered sugar just to kill the slightly bitter sour cream taste. This recipe was devised by a British war bride who was missing home. It has been hailed as the closest to Clotted Cream you are likely to get this side of England. Excellent over scones with strawberry jam.