To make the cucumber mix, add cucumber, simple syrup, lime juice and mint leaves to a blender, and blend until smooth. This is enough mix for 12 - 14 martinis.
To make the Cucumber Martini, add vodka and cucumber mix to a shaker with ice. Add a dash of agave nectar, salt and Tajin, cap the shaker, then shake until well-chilled.
Add 1 teaspoon of Tajin to a small plate with a small pinch of kosher salt. Wet the rim of a chilled martini glass with tamarind syrup. Turn the wet rim through the Tajin-salt mixture until evenly coated.
Strain the drink into the prepared martini glass, and garnish with cucumber slices.