Mix together the salt, sugar, cayenne pepper and lime zest in a small shallow bowl. Run a wedge of lime around the edge of each glass, and dip into the salt/sugar mixture. Fill each glass about half full of ice.
Add the lime juice, Mango Serrano jam, tequila and mango juice to a cocktail shaker. Stir to dissolve the jam.
Add ice to the shaker and shake to combine. Strain into the ice filled glasses, and garnish with a slice of serrano pepper if desired.