He'd really wanted to come in the armor. Just for the principle of the thing; it was his party, and he'd fly in armor if he wanted to. But he couldn't figure out a way to handle cake over that long of a flight without it ending up as mashed cake. Which, given the box around it, would suck. And then he'd have to ask for a screwdriver once there to pop out the manual release, and he wasn't sure that Bruce had anything as simple as a screwdriver. And while he was sure Superman could pry him out, he didn't want to think about what the armor would look like after.
So, he took a fast, sexy car and sped all the way to Gotham and up to Wayne Manor, the cake sitting in the passenger seat next to him. He was sure that would make an interesting stop sometime, if anyone actually dared stop him.
He was looking forward to this, and not for any elaborate reason. He didn't feel entirely guilty that he'd sent Pepper home as something as a birthday gift from him - he liked her, he'd been glad to spend the day with her, but it was almost because of that he'd sent her home. It was something nice to do for her, making sure that she wasn't waiting up and worrying, because he actually wasn't doing anything stupid.
Bruce needed him. That's a lot of what it amounted to. Bruce had been kicked in the balls, and kicked a few more times once he was down for good measure. Even if he wanted to brood alone and in pain, he should have people who cared about him around.
He brought the cake in, and had it intercepted by Alfred before he could protest that he could just take it down...actually, he also needed Alfred to direct him to the entrance to the Batcave, but he resented, just a little, the fact that it had cost him his cake. At least until Alfred brought it down to them.
He'd only been in the Batcave a few times, but he found his way towards the sound of the voices. At least the kids weren't here, which he was sure would just add another layer of awkwardness. If this could be about three guys having a drink and cake and bullshitting, he'd call it a win.
"Party's here!" he announced cheerfully as he wandered up. "Except I had to leave it with Alfred and then he took it into the kitchen. I'm assuming he'll bring it down in due time. On a silver platter."