To a cocktail shaker, add gin, blueberry syrup, lemon juice and egg white. Add on the lid and shake vigorously for 15 seconds.
Add in ice and shake the mixture again until it is thoroughly chilled.
Remove the ice from the coupe glass, then strain the drink into the chilled glass.
Serve with a toothpick speared with fresh blueberries.
Notes
Blueberry syrup: To make the blueberry syrup: combine 1½ cups blueberries, 1 cup sugar, 1 cup water, and 1½ tsp lemon juice in a saucepan. Simmer 10 minutes, cool to room temperature, then strain. Full recipe at blueberry simple syrup.
The dry shake (shaking without ice first) is essential for creating the silky foam layer.
Make your blueberry syrup ahead of time so it's completely cool.
For a vegan version, substitute the egg white with 1/2 oz (15 ml) aquafaba (the liquid from a can of chickpeas). It froths up just as nicely.
Try swapping the blueberry syrup for raspberry syrup for a raspberry gin sour variation.
One lemon yields approximately 1 oz (30 ml) of juice.