Put some agave in a small dish and the black lava salt in another dish. Dip half of the rim of a cocktail glass in the agave and wipe away the excess. Dip the same half into the black lava salt and fill the glass with ice.
Put the tequila, lime juice, agave, mezcal and egg white if using into a mixing tin, add large ice and shake vigorously. Immediately single-strain over the ice in the prepared glass. Garnish with the lime wedge and serve.
Eggs whites have been used in cocktails for hundreds of years. While they do not add significantly to the flavor, they provide a wonderful creamy texture. For those concerned about food-borne illness, use pasteurized egg whites from a small carton. Always check the expiration date and once the carton is opened, do not reuse.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.