1. Place tamarind in small pot with water over medium heat. Let it reach boil with low heat.
2. Remove tamarind from heat and let pulp set for 30 minutes. Pour puree through a fine-mesh strainer, lightly pressing on it with a spoon, atop a bowl. Let cool.
3. Add simple syrup into a mixing glass and lightly mash two serrano chile slices inside.
4. Prepare glass by coating rim with lime juice and dipping rim in seasalt.
5. Add remaining ingredients inside mixing glass, fill with ice, and shake for 10 seconds.
6. Strain over ice inside salt-rimmed glass and garnish with lime wedge.