I would understand if it were chocolate. Maybe death by chocolate?
Mark on her neck suggests that she was asphyxiated. No reflection in the mirror indicates that she is a ghost. Judging on the male's hand in blood she did her vengeance already while he was trying to eat some cake. Kinda complicated story...
I missed the mirror and the neck (it doesn't show up enough to tell on my display). I'm not an expert, but shouldn't the knife have some leftovers from the work (cutting the cake, offing the thief or a little of both)?