Pour Kahlúa, lemon juice, simple syrup and egg white in a shaker. Shake for ten seconds. Then add a big scoop of ice and shake again. Strain into an old fashioned glass filled with ice. Garnish with a lemon wedge. Try replacing the egg white with aquafaba. Aquafaba is the viscous water in which legume seeds such as chickpeas have been cooked in. It makes a really nice foam. A good idea is to freeze aquafaba in an ice cube tray, then melt it when needed. That way you will always have some at home to create a nice foam for your drinks. Around 20 ml or 2 ice cubes can replace an egg white.