Step 1In a blender, combine the 40 blackberries, sugar and lemon juice and purée. Strain, removing any seeds and particles.
Step 2Place the purée with the cognac and ice in a large cocktail shaker and shake to chill for about 30 seconds.
Step 3Pour the mixture into a punch bowl and add ice. Top with sparkling wine, use a ladle to lightly stir, and garnish with plenty of blackberries and lemon wheels. Serve immediately.