Work all the ingredients for the dough together and leave to rise for 40 minutes. Mould the dough by hand into 4-5-cm balls and fry in hot oil. Drain and cut into halves when cool. Mix the yoghurt, lemon and salt together to make a dressing. Serve the doughnuts with a teaspoon of dressing and a teaspoon of caviar on top, sprinkled with some chopped dill.