This is an absolute classic. I was first introduced to it by Joe McCanta, the brand ambassador for Grey Goose, when he did an excellently boozy masterclass at Shoreditch House. It's such a simple cocktail and tastes just incredible - really refreshing and actually kind of like champagne in a weird way and not at all like a vodka and soda.
You will need:
One shot elderflower cordial
One or two shots of Grey Goose vodka
Squeeze of lime
Shake all the ingrdients bar the soda together with some ice. Strain into a champagne flute and top up with soda water. Try not to drink it all at once.