Pastel de nata is a small egg-cream tart common throughout Portugal, although they are also known as pastel de Belém because they are said to original from a famous bakery in the Belém area of Lisbon. The plural is “pastéis de nata” or “pastéis de Belém”.
The most famous and sought after pastéis de nata are sold at the eponymous shop Pastéis de Belém. The natas sold here are made using the original secret recipe concocted at the Jerónimos Monastery nearby. So protected is the secret recipe that only three or four of the family of bakers are said to know the ingredients. Some speculators think the difference is in the cream, others think it is in the pastry shell.
You can go to any café in Portugal and ask for a pastel de nata, but only in the Pastéis de Belém shop will you get the extra special pastel de Belém.
If you want to learn how to make the generic ones, here’s a video.