Chill a coupe glass.
Combine the gin, Grand Marnier, sweet vermouth and lemon juice in a mixing glass.
Add ice and stir for 20–25 seconds until well chilled.
Strain into the chilled coupe.
Express a lemon twist over the surface and drop it in.
Harry Craddock mixed this at the Savoy Hotel for the leap-year celebrations of 29 February 1928, and claimed in print that it had been responsible for more proposals than any other cocktail ever mixed. Grand Marnier rather than plain triple sec is the point — the cognac base is what gives it weight.
My Bar Shelf läser ditt skåp och visar varje cocktail du kan göra av det du redan har - inklusive Leap Year.