Add the mint leaves, lemon juice, syrup and bitters to a shaking tin and muddle.
Add the cognac, and fill with ice.
Shake and strain into a Collins glass.
Fill with crushed ice and top with Champagne.
Garnish with a mint sprig.
*Black-tea–demerara syrup: Add 2 black tea bags to 1 cup of boiling water and steep for 3 to 4 minutes. Discard the bags, and stir in 1 cup of demerara sugar until dissolved. Let cool; the syrup will keep in the refrigerator for up to 2 weeks.