In a small pot over medium heat, dissolve the sugar in 1 cup water. Bring to a boil, add the lavender, and remove from the heat. Let sit until cool and strain. (The simple syrup will keep for 2 weeks in the refrigerator).
When ready to serve, in a pitcher stir together the vodka, cassis, and half the lavender simple syrup. (You can adjust the sweetness level to your liking). Chill each martini glass with ice until well chilled and remove ice, filling them about 3/4 full with the lavendertini. Top with sparkling water and garnish with fresh lavender sprigs.