Step 1

In a cocktail shaker, muddle the serrano pepper (reserve one slice for serving) with the lime juice, about 30 seconds

Step 2

Add tequila, triple sec, cilantro and bitters

Step 3

Fill the cocktail shaker with ice, cover, and shake vigorously until the outside of the shaker is very cold, about 30 seconds.Place the Tajín on a small plate

Step 4

Rub the rim of a rocks glass with the lime wedge, then dip the glass rim into the Tajín to coat

Step 5

Fill the glass with ice and pour in the margarita

Step 6

Garnish with the lime wedge and reserved pepper slice

Step 7

Serve immediately.