Chop the chocolate into small pieces and put into a bowl.
In a medium saucepan, bring 2 inches of water to a simmer. Remove pan from heat. Set the bowl of chocolate over the hot water, stirring frequently until just melted. Remove bowl from hot water. Cool.
Bring the water in the saucepan back to a simmer.
In a large bowl or in the bowl of a standing mixer, combine the egg yolks, sugar, and creme de menthe. Set bowl over simmering water and whisk constantly until egg mixture is slightly thickened, about 3 minutes.
Remove bowl from hot water. With mixer on medium speed, whisk the egg mixture until light and cool, about 4 minutes.
In a small saucepan, bring about 1 inch of water to a simmer.
Put 1/4 cup of water in a small bowl, sprinkle with gelatin, and let stand until softened, about 2 minutes.
Remove pan of simmering water from heat and set bowl of softened gelatin in hot water until gelatin has melted, about 2 minutes.
Whip cream to soft peaks.
Whisk gelatin into cool egg mixture, and then divide egg-gelatin mixture evenly between two bowls.
Fold 1/2 of the whipped cream into one of the bowls. Stir melted chocolate into second bowl and then fold in remaining whipped cream.
To layer the mousse, divide 1/2 of the Chocolate Mint Mousse between six individual goblets. Spoon all of the mint mousse on top.
Make a final layer using the remaining Chocolate Mint Mousse.
Refrigerate until set, at least 2 hours.
Recipe can be prepared and refrigerated 1 day ahead.