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. Garnish with remaining raspberries and serve.
I like this recipe because there are a lot of fresh flavors that come together. You can make it sweeter by adding more sugar, fresher by adding more raspberries and mint and stronger by adding more vodka. In that way, it is a cocktail that becomes unique to your own taste. I also really like to use superfine sugar because it dissolves more easily.