Chill a cocktail glass in the freezer or with ice water.
Add cognac, triple sec, and fresh lemon juice into a cocktail shaker.
Fill the shaker with ice cubes.
Shake vigorously for 10–15 seconds until well chilled.
Double-strain into the chilled cocktail glass.
Garnish with a lemon twist, optionally running it around the rim before dropping it in.
The ratio debate is central to the Sidecar - the 5:2:2 build used here leans spirit-forward and drier than the equal-parts French school. A sugar-rimmed glass is traditional in some bars but divides purists; serve it plain to let the cognac shine.
My Bar Shelf reads your shelf and shows you every cocktail you can make from what you already own, including Sidecar.