Place the black sugar or charcoal sugar on a small plate. Run a lime wheel around the rim of a chilled coupe or martini glass, then dip the rim lightly into the sugar to coat. Set glass aside.

Dry shake (if using egg white)

In a cocktail shaker, add blanco tequila, mezcal, fresh lime juice, fresh lemon juice, blackberry liqueur, simple syrup, and the egg white (if using). Seal the shaker and dry shake vigorously (no ice) for 15–20 seconds to emulsify and create foam.

Wet shake

Open the shaker and add a generous handful of ice. Seal and shake hard for 12–15 seconds until the shaker is well chilled and slightly frosted.

Double strain and finish

Double-strain the cocktail through a fine mesh strainer into the prepared chilled glass to remove ice shards and any large foam bubbles. Add 2 dashes of Angostura bitters on top of the foam; use the tip of a toothpick to create a simple design (optional).

Garnish

Skewer one fresh blackberry (or thread two) and rest it on the rim or drop it gently into the cocktail. Float the lime wheel on the glass edge or place it beside the garnish.