For sailors in the 1700 and 1800's, a Whiskey Sour was considered the best a man could drink. In a whisky sour, the lemon juice is (almost) as vital as the bourbon. A generous, foamy top is what you're aiming for.
100g soft light brown sugar
50ml whisky or bourbon
25ml lemon juice
1 egg white
Put the sugar and 100ml water in a pan and heat gently, stirring, until the sugar dissolves. Cool.
This will make more than you need but will keep in the fridge for a couple of weeks.
Put the whisky, lemon juice, egg white and 25ml of the brown sugar syrup into a cocktail shaker filled with ice and shake vigorously for 15-30 seconds.