The entire thing is Grunt's idea to begin with.
Right after his rite of passage or whatever the hell the krogans call taking down an actual real live thresher maw besides, let's face it, a pretty awesome way to deal with puberty, he takes it into his head to look up his new Battlemaster on the extranet. He's pretty lucky Shepard's still kind of the face of the Alliance, dead or not, and at some point before he gets sidetracked and starts looking up pictures of sharks, he finds out about human birthday customs.
He keeps it to himself for a while. He's always been smarter than he lets on. But by the time he finally figures out when April 11 would be if time on the Normandy made any fucking sense at all, it's already the end of March. Or probably the end of March. Calculating what the date is on Earth when you live on an FTL-capable ship is pretty damn tough.
Jack drops by to chat later that week, because sometimes you just want to talk about tearing shit apart and you won't find anyone better than Grunt when you do. So that's when he tells her. And of course she thinks it's the stupidest thing she's ever heard of in her life, and it devolves pretty fast into a headbutting contest which she narrowly wins because of her biotics. But he still wants to do it, and he says he'll do it himself if he has to, because Shepard's the best Battlemaster anyone could ask for, and she really doesn't think the galley could handle a teenage krogan all by himself.
And okay, if she's perfectly honest with herself, she kind of thinks Shepard's exactly the kind of girl scout who'd appreciate it. And the way Jack sees it, she kind of owes Shepard one. It hasn't been that long since Pragia.
"Okay," she says. "Fine. Whatever." And Grunt beams, actually beams at her, until she punches him for weirding him out.
They're in luck that week, because Shepard decides to drop by the Citadel for repairs. Jack's no tech, and Grunt's not exactly a master of subtlety, but together they manage to hack Gardner's usual shopping list to include a few more eggs and a little more flour, and somehow nobody notices. And Gardner always turns in early, so the mess is pretty much abandoned on the night of the 10th. Easy as anything.
The actual baking part is worse.
Spending your formative years as a Cerberus superweapon means you don't see a lot of cakes growing up. And Grunt spent his childhood in a tank, so even if krogan did do the whole birthday thing, he'd be shit out of luck, too.
Grunt tears the bag of flour clean in half on his first try. Somehow Jack manages to be even worse than Grunt when it comes to cracking eggs. The cake, when it finally makes it into the oven, promptly catches fire. And just about when Jack is cursing herself out for the tenth time in a row for thinking this was in any way a good idea, and Grunt is dumping any liquid he can find on the cake in a vain attempt to put it out, Zaeed Massani walks in.
"What the fuck are you two doing?" he says.
About a hundred less-than-witty comebacks spring immediately to Jack's tongue, ranging from Did we wake you up, old man? to Fuck off, but Grunt beats her to the punch.
"I baked Shepard a cake!" he announces in the exact same tone of voice he used to tell her he and Shepard had killed a thresher maw. "Because it's her birthday!"
The merc stares at them for a moment, his mismatched eyes moving from exploded bag of flour to eggy krogan to gently smoldering cake.
Jack decides that getting judged by Zaeed of all people is really fucking weird.
Then he starts to laugh, and she figures she should be mad, but she can't really. Not when he looks at her all deadpan and says, "Didn't anyone ever tell you not to overmix your guddamn batter?"
"Whatever," she says. "If you're so good at it, you might as well lend a hand."
"This takes me back," he says as he saunters over, and she flicks a crunchy brown thing at him to shut him up.
As it turns out the old fart's a pretty good cook for someone who looks like he spent the last twenty years eating shit out of cans. Hidden depths, or something like that. Whatever. Point is, by the time the artificial morning rolls around, they've managed to make something that has a slightly smaller chance of killing Shepard than the same cake except dextro. By this time EDI has figured out the whole thing, because how could she not, and—in a rare display of it's a goddamn miracle—actually unlocks Shepard's cabin door so Jack can slide the thing onto her desk while Zaeed and Grunt watch her six.
For a moment they all just kind of stand there, looking at the cake and feeling pretty stupid. But it's a good kind of stupid, like they managed to pull off something without a hitch for once.
"Shepard will love it," says Grunt, and Jack cuffs him on the head.
"It still looks like shit, dumbass," she says. But privately, she thinks he might be right.