Stir vodka, Cointreau, lime juice, and cranberry juice in a pitcher to combine. Do ahead: Cosmo mix can be made 10 hours ahead. Cover and chill.
To make 1 cocktail, fill a Collins glass two thirds full with ice. Pour in 2 oz. club soda and 2 oz. cava, then 3¼ oz. Cosmo mix. Add a dash of bitters and gently stir once. Garnish with a rosemary sprig.
