Optional: brush glass rim with syrup or lime juice and dip in Tajin or a mixture of coarse salt and chili powder to coat rim.
In an ice-filled cocktail shaker, combine tequila, Cointreau or triple sec, mango puree, jalapeño syrup, lime juice.
Shake well and strain into prepared glass filled with ice.
Garnish with lime slice and sliced jalapeños, if desired.