Combine gin, light cream, powdered sugar, fresh lemon juice, and egg white in a shaker without ice.
Dry shake vigorously for 15 seconds to emulsify the egg white and cream into a stable foam.
Add a full scoop of ice and shake hard for another 15 seconds until well chilled and silky.
Double-strain into a chilled highball glass without ice.
Top slowly with chilled soda water, stir once gently with a bar spoon, and serve immediately.
Serving a fizz in a highball glass without ice is traditional - it preserves the delicate foam crown and allows the soda to integrate cleanly. Pouring soda slowly down the side of the glass prevents the egg white foam from collapsing.
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