1. Melt the chocolate in a double boiler over a simmering (not boiling) water. While the chocolate melts, prepare a custard sauce:
Heat the milk just until scalding, when small bubbles appear around pan sides; do not boil.
2. Sprinkle the gelatin over the cold water in a small sauce pan and set aside for 2-3 minutes to soften.Stir over low heat until gelatin is completely disolved; do not boil. Rub a drop between your fingers to test for smoothness.
3. In a mixing bowl, whisk together the egg yolks and sugar until thick and light. Whisk about half the hot milk into the yolks, beatig constantly so the yolks do not cook, then pour the warmed yolks back into the remaining hot milk, whisking hard. Heat the yolk-milk mixture over low hear, stirring constantly until the custard thickens enough to coat the bacl of a spoon; you should be able to draw a line through the coating with your finger and leave a mark that does not close up. Do not boil the custard or it will curdle.
Check the gelatin; if thickened, stir gently over low heat just until liquified. Strain the custard into a bowl, then whisk in the dissolved gelatin and vanilla extract.
4. Little by little, whisk the melted chocolate into the warm custard, blending completely. Whisk in the creme de menthe and extract.
To thicken the filling at this point, you can refrigerate it for about an hour, stirring every now and then, until it begins to feel thick, mound of the spoon and look like a soft pudding. To speed the process, you can set the bowl of custard into a larger bowl of ice water and stir off and on for about 25 minutes, until it looks like a pudding. If the custard sets too hard, stir it over a pan of very warm water just until soft and smooth.
5. Using a chilled bowl and beater, whip the heavy cream until soft peaks form, then fold into the chilled and thickened chocolate filling. Pour the mixture into a bowl or individual dessert goblets and refrigerate at least 4 hours before serving plain or spreading between cake layers. Chill filled cake before serving, and refrigerate leftovers.