On late Sunday mornings, Bilbao locals hit the bars of the Casco Viejo neighborhood in search of tortillas, anchovies en salazón, Biscayan cod, cheese crêpes and other diminutive yet delicious pintxos that have epitomized the food culture of the largest city in the Basque region. Each venue has its own specialty, an ideal way to sample a variety across different bars. But when it comes to drinks, almost all patrons have the same thing in their glass: a Marianito.

A vermouth-based cocktail ubiquitous to northern Spain, the Marianito combines sweet vermouth, gin, Campari and bitters, served up or over ice. The drink is extremely popular in Bilbao, where it was supposedly created at a traditional bar before conquering the cocktail lists of high-end venues across the city. Legend has it that a waiter at a local bar, who used to drink vermouth in a small glass, sometimes with a splash of Campari or gin, fell in love with the daughter of someone named Mariano. When his feelings weren’t reciprocated, his friends began to call his drink of choice, which is essentially improved vermouth, “Marianito” as a joke about his former future father-in-law—and the name stuck.

Improved vermouth (or vermut preparado, in Spanish) is an apt description of the drink. Some argue that it is more of a proper cocktail, while others insist that it lies closer to an aperitif. “It is more like an Aperol Spritz,” says Marco Backo, an Italian head bartender who serves the cocktail at The Blind Pig, a speakeasy about 100 kilometers (about 60 miles) from Bilbao, in San Sebastián. “It is a slightly lighter drink, with a lower alcohol content. It’s vermouth with a twist,” he says.

Although it is popular for aperitivo hour, many drink Marianitos in cocktail bars at night, too. Based on its three main ingredients, the drink could be considered a vermouth-heavy Negroni. “But as the proportions change at every single bar, it always [results] in a very different cocktail,” explains Álvaro Fernández Marchante, founder and head bartender of Bilbao’s La Mula de Moscú.

Because each bar prepares the Marianito in its own way, recipes vary. One variable is the garnish used, which could range from an olive to an orange peel—sometimes both. Other additions to the mix can include Angostura, pomelo juice or olive brine. “The slight variations is what makes Marianito so interesting,” explains Marchante. 

There are also high-end versions of the popular drink. Mugaritz, a two-Michelin-starred restaurant in the Basque town Errenteria, lists the cocktail on its elevated beverage list. The preparation blends homemade vermouth, patxaran (a sloe-flavored liqueur) and aged rum in place of the expected gin. “We seek to make a version with more character [that distinguishes] between a Marianito and a Negroni,” explains Kristell Monot, the restaurant’s head sommelier. 

However, most of its variations stick to its prosaic heritage—with a few tweaks. Marchante created the signature Marianito he serves at La Mula de Moscú by upping the Campari to balance the sweetness of the vermouth. “I wanted to make bitterness more present,” he says. He maintains a splash of London dry gin and adds pomelo juice and two dashes of Angostura orange bitters for a crowd-pleasing drink. “On Sundays, we open early to maintain the Marianito tradition,” he says. “But on other days, I can guarantee the cocktail is also a success.”