Make the Filling: In a medium bowl, whisk the egg yolks until foamy, about 30 seconds. Stir together the gelatin, sugar, ½ cup of the heavy cream and the salt in a medium saucepan and let sit until the gelatin softens, about 5 minutes. Cook over medium heat until the gelatin dissolves and the mixture is very hot but not boiling, about 2 minutes. Whisking continuously, slowly drizzle the gelatin mixture into egg yolks. Return mixture to saucepan and cook, stirring constantly, until slightly thickened, about 2 minutes. Remove from the heat and stir in the crème de menthe and crème de cacao. Pour into a large bowl and refrigerate, stirring every 5 minutes or so, until it thickens to a wobbly consistency, about 20 minutes.