For the Rose Simple Syrup:
In a small pot, bring water to boil. Turn off the heat and add the roses. Cover and steep 3-5 minutes.
Strain out the roses and return the infusion to the pot. Add the sugar and, over low heat, stir until just dissolved. Remove from heat and allow to cool.
Store in a glass container with an airtight lid and refrigerate until needed. The syrup will keep for about 3 weeks.For the Cocktail: Combine sumac and sugar on a small plate. Rub rim of cocktail glass with lemon, then dip glass in sumac/sugar mixture to coat rim.
In a cocktail shaker filled with ice, combine gin, lemon juice, cardamom bitters and simple syrup. Shake well, then strain into cocktail glass.
Top with champagne.