these eggy, sugary treats are everywhere in Portugal.

Here, here, and here.

Some say the Pastéis de Belém is the original

pasteis de nata, or custard tart.

In this little factory, through this kitchen,

there's a recipe that has been kept top secret

for almost 200 years.

- Not this one, guys.

You can't get in here.

I'm sorry.

Okay, I'm Miguel Clarinha, and I'm part of

the management of Pastéis de Belém.

This special recipe you will only find here, at this shop.

We sell an average of about 20,000 tarts a day.

In the summer, maybe around 40,000 cakes.

A lot of cakes.

- [Narrator] Pastéis de Belém, as they call them here,

are simple tarts, really: egg yolk, milk, some flour, sugar,

but it's how they make them in complete secrecy

that makes these tarts a national treasure.

- We have six people knowing the recipe.

In the family, my father, my cousin, and myself.

We all trained the three chefs,

which is Vito Artur Domingos, Yodizel Janeiro,

and Carlos Martins.

- [Narrator] The recipe is so secret,

all the bakers sign nondisclosure agreements

to keep the timeless tarts a true mystery.

- Pastéis de Belém is one of the earliest recipes

at least here in Lisbon.

We know that the recipe was invented

in the Monastery of Jeronimos in the early 19th century.

This one.

- [Narrator] Once they have the recipe, the chefs,

now sworn to secrecy, are given keys to this door.

- They work inside the factory in a separate room

that we call the oficina do segredo in Portuguese.

It's like the secret shop or secret room.

They have to work in a separate room,

because the process can only be seen by them.

’Til today we haven't found anyone that explained the recipe

as it is.

If it happens, well, we probably would have to lock him up

in the basement and throw away the key.

(laughs) No, just kidding.

- [Narrator] Okay, secret-keeper.

What's the best way to eat one of these custard tarts?

- Usually I like it simple, but that's just me.

Most people like with a little bit of cinnamon on top.

This is the hard part of the job.

- [Narrator] Yeah, this is super hard.

I don't know how you do it.