In a cocktail shaker without ice, add tequila, lime juice, agave nectar and egg white. Shake vigorously for about 15 seconds to build the foam (this is called a dry shake). Add ice and shake again until well chilled and frothy, about 15 more seconds.
Strain into a rocks glass. Dot the frothy top with bitters. Use a cocktail pick to draw a line through the middle to create the heart-shape design (optional).