In a collins glass with a few ice cubes, pour the dry vermouth and crème de cassis.
Fill the glass with club soda and stir again.
Serve and enjoy!
Often, people prefer to leave the vermouth cassis unstirred. It produces a layered cocktail similar to the tequila sunrise—which originally used crème de cassis—that the drinker can stir with a straw if they like.
With this much dry vermouth, it is vital that it's fresh. Unlike distilled spirits, fortified wines have a short shelf life of just a few months. An open bottle will begin to go stale after that period and should be replaced.
The crème de cassis should not be an afterthought, either. Look for top-shelf options like those from Giffard, Gabriel Boudier, and Lejay, all of which produce authentic French cassis. It's also a flavor that some small distilleries, including Clear Creek Distillery and Tuthilltown Spirits, have in the liqueur portfolios.
This drink should be customized to suit your personal taste. Pour as much vermouth as you like—some recipes use up to 4 ounces—and sweeten it with crème de cassis as you see fit.
Seltzer is a common substitute for club soda. If you want to give the drink a drier profile, pour tonic water.