It was sheer luck that Crowley was in Spain at the time.
Aziraphale was up in England somewhere, having wandered in with the Roman missionaries and never really left. But it wasn't Crowley's lookout where the angel was; there wasn't really much that could be made holy in that foggy little dump, and so much the better if Aziraphale wasn't down in Spain thwarting whiles. It was all part of the Arrangement, which had been working for nearly five hundred years now. Crowley liked it. And didn't really miss Aziraphale too much at all, because interesting things were happening in Spain.
For example, there was a man named Cortez. Cortez had a vision. It wasn't a divinely inspired one, but purely human, and that made it dangerous.
Crowley hadn't received any instruction from Hell in quite a few years. He didn't get instructions very often, really; humans got themselves damned quite well enough without Crowley's having to do anything, and he'd worked out that he was mainly on earth for show. And because he'd been there since the Beginning. And because he liked it, but he wasn't about to mention that.
At any rate, his lack of instructions from Hell meant that Crowley could do exactly what he liked with his time, and at the moment this meant watching Cortez. More than watching, really; Cortez was bound for the New World, and Crowley was feeling rather morbidly curious about what such an ambitious and righteous man would do when he encountered the people living there.
Actually, Crowley could guess. And it wasn't his job to stop it. But he might as well be there, for the look of the thing.
Mexico was delightfully warmer than Europe. Crowley basked in the heat of it, and even turned serpent for a few days until a disgruntled python tried giving Crowley a piece of its small, reptilian, deeply annoyed mind, at which point Crowley switched back to man-shape and snuck in again among Cortez's men. He didn't like them very much; none of them knew how to have a properly good time, and it didn't help that Crowley had been stuck with them for far too many days aboard a cramped ship; Crowley hated ships. But he liked Mexico, so it was, more or less, worth it.
It was especially worth it when Emperor Montezuma invited them all to his palace and handed them great golden goblets. When Crowley received his, he peered into it skeptically; it looked very much as though it was filled with mud, and Crowley wouldn't have been surprised if it had been, though if this was so, he was ready to label Montezuma an idiot on the spot. He was also quite ready to think of the emperor as a genius if the man had thought to give them all poison, but a quick touch of Crowley's fingertip to the drink told him that, though this liquid was entirely new to him, it was not in any way fatal.
So Crowley took a sip.
He almost dropped it. Instead, he clutched at his goblet, and stared first at the unassuming muddy liquid, then around at the men around him, who were tasting theirs cautiously. Most of them were making faces and setting it aside.
Crowley wasn't surprised. The drink tasted dark, and rich and bitter and surprising.
It tasted like sin.
It tasted like pure sin. Contrary to popular belief, there is such a thing as pure sin; that is, sin completely undiluted, in its truest form. Very few mortals fall for pure sin; in fact, only megalomaniac villains of the sort found in comic books are attracted to it. Ordinary mortals needed… sugar, spices, warmth, and a friendly salesman, if they were going to buy sin.
As Crowley stared into his cup of this amazing, sinful, ordinary-looking drink, he had a sudden idea.
Upon their return to Spain, Cortez's men brought with them, quite proudly, chocolatl sweetened with cane sugar. It certainly tasted better this way, Crowley thought, but it still tasted more like sin masquerading as cane sugar than something dark and insinuating and delicious.
Crowley began to research spices. There had to be something that could make it taste the way he knew that it could. He was not, of course, a demon possessed, because the whole idea of such a thing was rather silly; all the same, he was certainly a demon driven, by his own fascination with the chocolatl.
And, after much trial and error, he turned up with a drink made with cocoa beans and sugar and cinnamon and vanilla, and he warmed it to just the temperature to make it delightfully stomach-warming and not quite scalding.
Crowley smirked at his creation of delightful, drinkable sin, and called it chocolate.
The Spanish were quite annoying, and protected the secret of chocolate jealously, determined that the rest of Europe should have no part in it. Crowley twiddled his thumbs (metaphorically, of course; it is very undignified to twiddle one's thumbs) and waited with growing impatience for someone to slip up and let the secret of chocolate out to the rest of Europe.
After about a hundred years, Crowley gave up and (with a silent, quickly stifled apology to Aziraphale) tempted one of the chocolate-making monks into letting the secret out.
He was intensely satisfied when chocolate proceeded to take the civilized world by storm. Chocolate was acclaimed as delicious and beneficial to health. For a while it reigned as the drink at the fashionable Court of France. Everyone loved it. Everyone wealthy drank it, and everyone poor had another reason to envy the wealthy. It was only to be expected, of course, but it was gratifying to see one's work so well accomplished.
Then chocolate went to Britain.
Feeling in the mood for a holiday, Crowley took a nonchalant flight up to London, and visited, with great delight, one of the new Chocolate Houses that had opened up there.
It was difficult to contain his amusement when he met Aziraphale in one of them.
"Hello, dear boy!" Aziraphale greeted him amiably, when Crowley slid into the seat opposite the angel, mug of chocolate in hand. "How have you been doing these past few centuries?"
"Wonderfully," Crowley returned, and gave Aziraphale a sharp grin. "Accomplished. How's the chocolate?"
"Quite good. Also apparently quite healthy, though I'm sure it tastes a little good to be that," Aziraphale returned, though without any trace of disapproval. "I'm quite sure it isn't divinely inspired. It isn't your side's work, is it?"
Crowley sketched a little bow. It was perfectly acceptable to take credit for his work when he knew it was a job well done.
"Really, my dear," said Aziraphale. "You're losing your touch."
Crowley arched an eyebrow. "Am I?"
"Yes," said Aziraphale comfortably. "I received a commendation for it, you know."
Crowley briefly considered feeling put out. Instead he laughed, and downed his chocolate. "Cheers, angel."