We use this trick with the flourless chocolate cake, or for any sticky cake. To heat your cake-slicing knife, either dip it in a tall container of very hot water or hold it under hot running water for a few seconds. Then wipe it dry before cutting the cake. The knife will cool quickly and the cake will start sticking, so expect to rinse and repeat several times. If you have a crème brûlée torch, it’s handy for heating up a knife, too.