Fill a rocks glass with 3 to 4 ice cubes, or one large cube if you have one.
Pour 45 ml (1.5 oz) Campari into the glass.
Add 45 ml (1.5 oz) sweet vermouth on top of the Campari.
Stir with a bar spoon for 15 to 20 seconds, until the glass feels cold and the two liquids blend into one deep red color.
Cut a wide strip of orange peel with a paring knife, avoiding the bitter white pith.
Twist the peel over the glass, skin-side down, to release a light mist of citrus oil onto the drink's surface.
Drop the peel into the glass and serve right away, while the ice is still intact.