Combine the sugar with 100ml water and gently bring to a simmer, stirring until the sugar is dissolved. Cool. This will make more sugar syrup than you need but will keep for a month in the fridge (visit olivemagazine.com for more cocktail recipes to use it up).
Put 40ml of the sugar syrup, the sherry, lemon juice and egg white in a cocktail shaker with ice, and shake vigorously for 30 seconds. Strain into 2 chilled coupe glasses.