In Philly, the Starbucks Unicorn Frappuccino may soon be rare as its mythical namesake. After unprecedented popularity in its first two days on the menu, several cafes around the city have already sold out.

“Good morning, we ran out of the Unicorn Frappucino, how can I help you?” said a barista answering the phone at the Starbucks on Grant Avenue and Academy Road in Academy Gardens on Friday morning.

“No, we’re not doing them today,” confirmed a staffer at the Temple University Starbucks. “We don’t have all the ingredients.”

“It was supposed to run five days, and we ran out in two,” said the manager of the 10th and Chestnut Starbucks, where most of the ingredients used to make the limited-edition special (which contains no coffee or caffeine) had disappeared by around 4 p.m. Thursday. The drink is slated to be available at cafes nationwide through Sunday, April 23.

Credit: Danya Henninger / Billy Penn

According to a customer at the 10th and Chestnut counter on Thursday afternoon, the 12th and Market Street Starbucks had also run out, which was why she’d walked over to this one. And also why she was settling for an imitation substitute dreamed up by the harried barista behind the counter — some extra pink powder he’d dug up, whipped into cream with vanilla syrup.

“I walked all the way down here; I might as well get something,” she said, as her boyfriend tapped his foot impatiently by the door.

Right. Unlike other specialties dreamed up by Green Mermaid drink designers — pumpkin spice lattes, for example, or toasted-coconut cold brew — the Unicorn Frappuccino is not drawing swarms for its flavor, which is a mix of mango (the pink base) and “sour blue” (whatever that tastes like). Customers are coming for the look.

“I saw it on Snapchat,” said Joey Chandra, who had nabbed himself a sparkle-dusted, whipped cream-topped cup at the 3rd and Arch location, and was busily tapping away on his phone to add it to his own Snap Story. Did he know what it was supposed to taste like? “Um, no idea.”

Credit: Danya Henninger / Billy Penn

“They’re ma-gic-cally delicious!” a barista named Brandon chimed in helpfully, as his coworker John rolled his eyes and shook his head in the background. Their store was “kind of off the beaten path,” they explained, so they hadn’t gotten as bombarded as some other locations.

Foster, a barista at the 12th and Walnut outpost, had fulfilled an order for 10 of the buzzy slushies earlier that day, he said. But that was nothing compared to some rumors percolating through the Philly Starbucks grapevine: “My friend who works at Broad and Jackson, she made 60 at one time!”

Indeed, buying the pretty drink en masse has become a thing. A barista in Colorado posted a (now-deleted) viral video rant on Twitter, complaining that “it is difficult to make when there are like 20 fraps all at once.” An anonymous post on Starbucks Reddit moaned: “Any other baristas feel like putting a severed unicorn head in [Starbucks Executive Chairman] Howard Schulz’s bed with a note saying “never again”?”

At the Starbucks inside the Bellevue, Dr. Paul Reuben, a chiropractor who was stirring his first Unicorn as he flicked through his phone at the counter, said he was inspired to get one after seeing a fellow chiropractor in NYC post photos of his whole office sipping purple together.

“Now I might have to buy them for my whole office,” Reuben said. He was a fan of the tangy-sweet flavor, which he described as similar to Sour Patch Kids or SweeTARTS.

“We definitely didn’t expect as many orders as we’ve gotten,” said Angela, manager of the Starbucks at 16th and Walnut, where supplies were still in stock Friday morning. She said her staff was enjoying the craze, and hadn’t gotten overly stressed. “Other cappuccinos are even harder to make.”

Danya Henninger is director and editor of Billy Penn at WHYY, where she oversees the team, all editorial decisions, and all revenue generation, including the membership program. She is a former food and...