Add the Campari, dry vermouth, lime juice, and simple syrup to a cocktail shaker.
Add ice cubes and shake until well chilled.
Strain into a Collins glass filled with ice.
Top with soda water, stir to mix, and garnish if desired.
Notes
To make a batch of 8:
1 cup Campari
1 cup dry vermouth
1 cup lime juice
¾ cup simple syrup
Soda water for topping
Mix everything except the soda water in a large pitcher. Chill up to 1 day in advance. Avoid adding ice directly to the pitcher.Divide the base amongst cocktail glasses filled with ice, and add the soda water individually just before serving to keep it fresh and fizzy. Tips:
For a sweeter cocktail, increase the amount of syrup or add a splash of orange juice.
If you want a more sour cocktail, increase the lime juice.
Use Aperol instead of Campari for a sweeter bitter orange flavor. I recommend reducing the simple syrup if you use Aperol as it is sweeter than Campari.