First make the raspberry granita: Place water and sugar in a bowl and stir to combine. Let sit for about 15 minutes, stirring occasionally, until sugar completely dissolves. (You’ll see how the mixture turns from white to a more translucent color as the sugar granules disappear.)
Meanwhile, place raspberries and lemon juice in a food processor. Add the sugar mixture and blend on high until completely smooth. Taste and add more sugar if needed.
Pour into a shallow freezer-proof dish (I useda glass 2 quart Pyrex dish). Cover with plastic wrap and freeze until almost firm, stirring 3-4 times in the first two hours.
Continue freezing granita for at least 3 more hours or overnight. Granita keeps for 3 days, keep it covered and frozen.
To make cocktails: Take granita out of the freezer and scrape the surface with a fork to form crystals. Scoop granita into 4 glasses.
Top with a small splash of elderflower syrup (optional), crushed ice, mint leaves and pour over the rosé. Add more ice if needed, a straw and raspberries for garnish. Serve immediately.
To make lighter cocktails you can use less rosé and add some sparkling water or even raspberry juice instead.