From Cook's Ill, uses a frozen egg white which they suggest as a leftover from their Runny Yolk Sauce.
Ingredients
2 fluid ounces (1/4 cup) whiskey (prefer bourbon or rye)
1 frozen egg white cube (about 1 ounce)
1/2 fluid ounce (1 tablespoon) lemon juice
2-1/2 teaspoons granulated sugar
Pinch kosher salt
1 cup ice cubes
Instructions
1. Combine whiskey, egg white, lemon juice, sugar, and salt in cocktail shaker and vigorously shake until egg white has melted and drink is very foamy, 60 to 90 seconds.
2. Add ice to cocktail shaker and vigorously shake for 30 seconds. Strain into chilled coupe or rocks glass and serve.