Step 1Combine the elderflower and water in a saucepan and bring to a boil over high heat. Remove from heat and set aside until the liquid is cool. Strain the elderflower and discard. Stir in the brown sugar until the sugar is dissolved. This makes a generous 1 1/2 cups syrup, more than is needed for the remainder of the recipe. The syrup will keep, covered and refrigerated, up to 1 month.
Step 1In a cocktail shaker, combine 3 rose petals, the lemon juice, elderflower syrup and gin with plenty of large ice cubes. Shake lightly, and strain into a Champagne flute. Top with Champagne and garnish with the remaining rose petal.