Calvados is an apple brandy famously made in the Normandy region of France. It's American counterpart is called Applejack, which in a pinch I suppose you could substitute, but only in a pinch. The Calvados Cocktail first appears in "The Savoy Cocktail Book", published in 1930 by Harry Craddock, the bartender at London's famous Savoy hotel. That recipe called for a lot more orange bitters, using essentially as much of that as there is Cointreau. I felt that was just a tad overbearing, so I've pulled it back to the more modest amount you see here. But feel free to kick it up a notch if you want!
Shake with ice. Strain into a cocktail glass.