Add the diced mango and serrano chilis to a cocktail shaker.
Use a muddler or spoon to mash or "muddle" the mango and serrano chili in the bottom of the shaker.
Add the tequila, triple sec, agave nectar and ½ cup ice to the shaker.
Shake well, then strain and pour into a glass filled with ice.
Spice Level: The amount of serrano chilis used in the cocktail can be adjusted to make the cocktail more mild or spicy. Using the whole pepper, including the seeds and membranes inside the pepper, will make the cocktail spicier.
Mango: make sure the mango used is ripe, meaning soft to the touch when gently squeezed, or it will be hard to muddle into the cocktail.