Combine the egg yolks, Champagne, and sugar in the top of a double boiler and whisk over simmering water (make sure the bowl does not touch the water in the bottom pot). Whisk constantly until thick and foamy. Remove to an ice bath and continue whisking until cool. Set aside.
Whip chilled heavy cream to soft peak stage and fold gently into custard. To achieve the most volume, plan to fold in whipped cream just before serving. Serve in chilled glasses layered with fresh fruit and/or sponge cake. Garnish with mint.