Smokeless powder, the slowest burning grade you can find for rifles, is probably best. Anything faster burning than that and you'll get an unimpressive CRACK (likely to unevenly blow the cake apart, unless you use way too much demo) instead of a cool-looking shove that actually moves the cake pieces apart.

You need to pack the powder to get any real effect from it, it can't just be loose (and anyway a cake would get it moist, and that's bad), so wrapping it tightly in paper or something similar is a good idea. You'll want a decent amount to get the right "completely blowing a cake apart" effect on video.

Since the material factors for your project are unknown (no way I can build a demo chart to effect based on "a cake", no reliable way to know how you're going to pack or place the device, and no idea what the device will be) you'll have to go with the Goldilocks approach: make three cakes and three devices. Make one you think is too small and try it out. Make one you know damn well is far too big and try it out. Make one based on the two sets and your newfound experience packing (and probably fizzling at least one) devices and you might get it just right.

Aside from the explosive itself you're going to need some high speed camera work. No matter what or how you do things this is going to be the single most important part of what you do. Even an underpowered device just shaking the cake would actually look pretty cool if slowed down enough in high detail -- so keep this in mind.

Anyway, that's my input on it. Not that I know anything about this -- I was an 18C (Special Forces engineer) for a decade before finally getting out and doing what I really wanted (SF wasn't bad, but with the wars winding down it sort of silly to stay in the Army!).

Oh thank you, i asked on a few other boards and i got similar info, although the smokeless input is new (and much appreciated). Would it be alright if i just used something like tissue paper to pack the powder? and should i lay it all in one thin sheet of paper, the width of the cake, or maybe little packets spread through out? As for camera stuff, I've got it down, didn't spare any expense on that portion, although i don't have the best of the best. Thank you for your input!

Not the guy you're responding to but try something a little more strong then tissue paper, as smokeless powder burns pretty slowly without somewhat of an enclosure (not much stronger, mind you, probably just fabric or something like it).

As for the position of the charges, I would say maybe one big 'un in the middle and maybe three smaller ones encircling it.
Also that picture makes me so god damn hungry for a nice cake