Combine sugar, chopped sage leaves and 1/2 cup water in saucepan over medium-high heat. Bring to a boil, then reduce and simmer until sugar is dissolved (2-3 minutes). Set aside to cool, then pour into airtight container and chill for at least one hour. Combine sage syrup with 3.5 cups water, lemon juice and vodka. Pour salt onto a small saucer, spread into a thin layer. Use half of a lemon to moisten the top of a tumbler or martini glass, then rim with Meyer Lemon Flake salt. Add ice and sage leaf to glass, then pour cocktail. Top with prosecco floater.