Melt chocolate in a double boiler or metal bowl set over a pan of simmering water, stirring regularly until melted. Remove pan from heat but keep bowl on top to stay warm.
Whisk together 1/3 cup cream, eggs yolks, sugar, and a pinch of salt in a small saucepan. Whisk over medium low heat, stirring constantly, until it registers 160°F on thermometer, about 3-5 minutes. Strain mixture into a bowl and stir in vanilla and melted chocolate until well combined. Let cool until to about room temperature (I alternated putting into freezer for a few minutes, then whisking since I was in a hurry)
Beat remaining 2/3 cup cream until stiff. Fold one fourth of whipped cream into chocolate custard to lighten, then fold in remaining cream gently but thoroughly.
Spoon mousse into 4 dessert dishes or glasses and refrigerate for at least an hour.
To decorate for Halloween, twist and removed center filling of 4 oreo cookies. Crush wafer sections to form the graveyard “dirt” and sprinkle on top of chilled mousse. Cut the Milano cookies in half to form 4 gravestones. Put buttercream in a baggie and cut the smallest hole you can. Write RIP on the cookie halves. Set a cookie gravestone and cany pumpkin in each mousse serving.
Notes
There are pre-filled decorating tubes that might make the cookie decorating (writing RIP) process easier.
I am not a health professional and nutrition data is calculated programatically. Accuracy may vary with product selection, calculator accuracy, etc. Consult a professional for the best information.