How To Make A John Collins

Has anyone had the opportunity to create this cocktail? I have an interesting recipe and instructional video for everyone that would like to try. It is a great refreshing cocktail for the Summer

2 PARTS Beefeater London Dry Gin

1 PART Fresh lemon juice

3 BARSPOONS Castor sugar

BUILD Ingredients into a Catalina glass over crushed ice and churn

TOP UP Soda water

FINISH With another topping of crushed ice

GARNISH With 2 lemon slices and serve with 2 straws

This sounds GREAT!! I LOVE Tom Colins, but I am wondering what is “Castor Sugar”? Once I find out, I will make this for sure!!