Fábrica da Nata is one of the best places in Lisbon where you can find and taste the traditional pastel de nata ("custard tart"), a Portuguese egg-based pastry, optionally dusted with cinnamon and icing / powdered sugar.
This chain has three shops in Lisbon, one in Sintra and two in Porto.
We also have a post on the Fábrica da Nata shop located at Rua Augusta, a great spot due to its balcony in the second floor.