The cookie-croissant mash-up was created at a posh boulangerie in Paris and is now being sold in London after the trend exploded on social media.
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Some 2,000 teenagers a day have been lining up around the block in the French capital to try one.
Pastry chef Stéphane Louvard said he has had to hire extra staff there to meet the demand.
The recipe is essentially a classic butter croissant sliced open and crammed with cookie dough, with hazelnuts sprinkled on top.
The overnight popularity has now inspired British bakeries to create their own versions.
Top French bakery Philippe Conticini has started flogging crookies in London for £5.90, with staff selling 100 a day at their South Kensington branch alone.
When The Sun visited for a taste test, all the crookies were sold out by lunchtime.
While the bakery was unable to say how many calories the treats contain, a normal croissant and cookie each contain around 500, suggesting the hybrid treat could top 1,000 — around half of an adult’s daily allowance.
The crookie is the latest in a string of hybrids, after croissant-doughnut mash-up “cronuts”, croissant-waffle “croffles” and croissant-muffin creations, named “cruffins”.