If croissant is to Paris, the pastel de nata is to Porto.  It's a puff pastry filled with egg custard that dates back to the 18th century.  Catholic monks used leftover egg yolks to make pastries like the pastel de nata to generate revenue. Although they can be found in almost every bakery, Casa das Tortas makes some of the finest egg tarts in the city.

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Pictured: Pastel de nata