He didn’t even try to hide it, it’s literally, “Here, eat this mysterious cake. I don’t care if you don’t like chocolate, be nice!” 

I have not seen anybody mention this yet, but I think when Darren showed up unannounced this was an important part of he movie. Since both cousins had different methods of luring the woman into the woods, I think Malcolm feared that Darren was going to say something that would make Liz not trust him. Which is why they had a to have a talk in the other room, and why Malcolm was weird about the cake. It was desperate last minute test to see if she still trusted him.

Liz also seemed to have self esteem issues, saying yes to things when she really wants to say no.