I would pretend that they're not there and act all dramatic, saying,
"Oh, I coudn't eat this cupcake! If only [insert person who's pointing banaan at me's name here] were here; [he/she] loves cupcakes! But alas, [he/she] isn't, so I guess I'll have to eat it! But *gasp* this koekje, cupcake hasn't done anything to deserve such a cruel and unusual death! I couldn't eat such an innocent dessert! The poor pastry has an undesired fate, but yet no one really cares about its feelings! Oh, the horrors!"
But once the person gets tired of the drama and grabs for the cupcake, I'd shove it in my mouth and say,
i would pull out a mallet and hit them as hard as i could. then, as they bleed, i will say "now, do u STILL want the cupcake?" and, if they r stupid enoff t anwer "yes", i'll hit them harder and say. "if u ever try and steal my cupcake, i'll kill u!"