In a small pot, bring the water to a boil over medium heat, and add 1/2 cup of the sugar. Shut off the heat, add the mint leaves and set aside to cool. Once you can smell the mint, after about 2 to 3 minutes, strain and set aside. Discard mint leaves.
In a large pitcher, combine and muddle 2 pints raspberries with 1/4 cup of the sugar and the lime juice. Stir to blend with a spoon, crushing the raspberries
Add the mint simple syrup and refrigerate.
When ready to serve, remove the pitcher from the refrigerator. Top off with Champagne and pour into champagne flutes.
