Fantastic custard tart available from most pastelarias around the city, although the original ones come from one shop in Belem (Antiga Fabrica dos Pasteis de Belem - you can spot it by the queues outside) where they call them Pasteis de Belem. Custard tart really doesn't do it justice, they are truly stunning and one is never enough.
At pastelerias across the city, or at Antiga Fabrica dos Pasteis de Belem, Rua de Belem 84-88, Belem
Or you could try Lisboa on Golborne Road in London. Equally amazing and without the carbon footprint.