Usually it's best to immediately knock out onto a cake, if you choose to use that method.
I suggest, however, that you harvest some of the cake. It's easy, and it permits you to both clean your fermenter and pitch the right amount of yeast - using the entire cake is
way too much.
First, clean and sanitize several glass jars. Mason jars work best, but I've used apple juice jugs, as well as Nantucket Nectars bottles. As long as they're glass with relatively tight-fitting lids, you're all right.
Second, remove the finished beer, leaving as little beer behind in the vessel.
Third, carefully skim the yeast bed with a sanitized stainless-steel spoon - a large soup-spoon works okay for this, though a bit time-consuming compared to a serving spoon - and scoop the harvested yeast into the jar/bottle.
Fourth, cap the bottle(s) and refrigerate. Since you're only going to be waiting a few days, the more technical stuff involved in yeast harvesting isn't really worth worrying about. You don't need to wash it, you don't need to do cell counts - you just need to get enough yeast out of the fermenter to pitch into your next batch.
To determine the appropriate amount of yeast to pitch, consult the
Mr Malty Pitching Calculator. Before doing that,
read JZ's essay on calculating how much yeast to pitch.
Cheers!