Notes
While any sweet red vermouth can be used, the most common choice is Spanish vermut rojo. However, Cinzano or Martini & Rossi are also frequently used in Spain. I find the small ratio of Campari boosts the smooth very sippable rojo's in a complementary way.
The recipe provided is the most common/traditional in its components and proportions. (A smaller volume 50 ml/10ml/10ml/2 dash is not uncommon.) Various flourishes, additions such as curacao, and changes such as juices can be made. A wide variety of local offerings are described in the attached Spanish article link.
History
The history of the Marianito (aka Vermut Preparado) cocktail and origin of the name are not well established. Per Ana Vega, the first printed mention of the name is from a 1989 newspaper, but older people claim the name was commonly used long before that, and has become a staple in the Basque region in recent decades. It arose out of earlier Spanish vermouth-heavy cocktails of related composition stretching back to at least the 1920's--some using a bit of curacao, including an early Spanish language "Vermout Cocktail" in 1905. These did not include Campari.
Some modern variations add pomelo or orange juice, but the basic recipe calls for only vermut rojo, gin, Campari, and Angostura bitters.
Spain has a tradition known as "la hora del vermut" where vermouth is consumed as an aperitif before lunch.
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