In a cocktail shaker, muddle the raspberries. Add the lime juice, gin, and egg white (if using). Shake until well blended and frothy.
Strain into two coupe glasses and top each with soda water. Garnish with a lime twist or fresh mint leaf, if desired.
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Notes
The egg white can be omitted and the cocktail can instead be served in a rocks glass filled with ice and topped off with soda water and garnished as directed above.