Heat 3/4 cup cream in a 1-quart heavy saucepan until hot.
Whisk together egg yolks, sugar, and a pinch of salt in a bowl until combined well.
Add hot cream and gelatin in a slow stream, whisking until combined.
Transfer mixture to saucepan and cook over moderately low heat, stirring constantly, until it registers 160°F on a thermometer.
Pour custard through a fine-mesh sieve into a bowl and stir in vanilla.
Melt chocolate in a double boiler or a metal bowl set over a pan of simmering water stirring frequently.
Whisk custard into chocolate until smooth, then cool.
Beat remaining 1 1/4 cups cream in a bowl with an electric mixer until it just holds stiff peaks.
Whisk one fourth of cream into chocolate custard to lighten, then fold in remaining cream gently but thoroughly.
Spoon mousse into small cups and let set then add the Oreo cookie.
Once completely set de-mold and add the two chocolate coins to the top as a garnish.