What does Misha do to Janina's birthday cake?

Misha sabotages Janina's birthday cake by adding a variety of unpleasant ingredients, such as salt and mustard, which ruin the cake's taste. This act is part of a larger conflict between the characters, highlighting Misha's mischievous and antagonistic nature. The incident ultimately serves to deepen the rift in their relationship and reflects the themes of betrayal and conflict within the narrative.