Eggses! And my 1970s hand mixer because beating 3 egg whites in the food processor is too much fuss. Takes ages (instead of 2 minutes with the little Philips) and entails lots of cleaning. Saffron and egg yolks are what makes the batter so yellow. #kulich
I honestly don't know why the recipe says to beat the egg whites to soft peaks at this stage, because the batter (will actually be soft dough when all the flour is in) is going to be mixed and beaten a lot yet, losing all the lift of the egg white.