Rim a double rocks glass with chamoy and Tajín, or run a lime wedge around the rim and dip into Sal de Gusano. Fill the glass with ice and set aside.
In a cocktail shaker, add tequila, lime juice, agave, and Cointreau.
Add ice and shake well until chilled.
Double strain into the prepared glass over fresh ice.
Top with cold Mexican beer and garnish with a lime wheel and optional jalapeños.