Chickpea water (aquafaba) makes an excellent substitution for egg white, it foams really well and adds an ever so slightly nutty lightness to the drink.
We also use lime juice instead of the classic lemon. Fresh lime peel is one of our key botanicals so we're always on the lookout for a way to use up our peeled limes (alternatively, we recommend Asian and Mexican cookery!).
How to Make
Drain your chickpeas and catch their water in a receptacle.
Shake all ingredients with ice (for about 15 sec, that's longer than you think!) and strain back into the shaker. Dry shake (without ice) and strain into an ice filled glass.
With thanks to the amazing Difford's Guide.
Make your own sugar syrup
Add sugar and hot/boiled water to a jar/bottle - 2:1 ratio (sugar:water). Stir to mix until the sugar has dissolved, let cool down/put in the fridge. Any sugar will do here (although we haven't tried icing sugar yet).