Place your serving glass in the freezer to keep your cocktail refreshingly cold.
In a cocktail shaker, combine the raspberries and simple syrup. Use a muddler or the back of a spoon to crush the raspberries into a juicy pulp.
Pour in the gin, lemon juice, and egg white (or aquafaba). Do not add ice yet.
Seal the shaker and shake vigorously for about 15 seconds. This step helps emulsify the egg white and create a thick foam.
Add a handful of ice to the shaker, then shake for another 15-20 seconds until the shaker feels icy cold.
Using a fine mesh strainer, double strain the cocktail into your chilled glass to remove raspberry seeds and create a smooth texture.
Top with a few fresh raspberries on a cocktail pick or a lemon twist.
Optionally, add a dash of Angostura bitters to the foam for extra flavor and visual appeal.