Pastéis de Belém, one of the original egg custard tarts, is a widely enjoyed dessert in Portugal; its namesake bakery has been known to sell an average of 20,000 tarts a day—and as many as 40,000 per day during the busy summer season. And yet, only six living people know precisely how these heavenly mini-desserts are prepared.

How exactly does one keep a recipe secret for more than 180 years? Let's just say the owners of Pastéis de Belém take this stuff very seriously: Bakers are asked to sign a nondisclosure agreement and prepare the dessert in a locked room, separated from the rest of the staff. Check out the full video from Great Big Story above—just don't expect to catch a glimpse of any secret operations. (You'll need a key for that.)

Banner image: Great Big Story/YouTube