Whip the cream until stiff, then mix with the yogurt and sugar. Put the mixture in a kitchen sieve lined with a permeable kitchen towel (preferably linen). Now hang the sieve in a larger pot, leaving space below so that the liquid can drain off. Chill for one night.
The next day, carefully turn the cream onto a plate and remove the kitchen towel. The cream now has a firmer consistency because of the drained liquid and looks like an igloo.