Combine the peas and the cooled Simple Syrup in a blender and puree until smooth.
Add 1 ounce of the pureed sweet pea mixture to each Champagne flute. Add 1/4 ounce of elderflower liqueur to each. Top off with some chilled champagne and a mint leaf for garnish.
Combine the sugar and 1/2 cup water in a small saucepan over medium-high heat. Bring to a boil and stir until the sugar is dissolved. Set aside to cool completely.