In a cocktail shaker filled with ice, add 1½ oz tequila, ½ oz lime juice, ½ oz jalapeño agave syrup and 2 oz mango juice. Shake and strain into a cocktail glass with ice. Garnish with a slice of mango and cilantro sprig.
To make the jalapeño agave syrup, place the jalapeños – along with all the seeds – in a container with the agave syrup. Top it up with 7 tbsp of hot water and stir everything together. Set aside for 6 hours, then strain through a fine-mesh sieve into an empty container. Seal and store in the fridge until ready to use – it will keep for up to a month.