To make the crust: Butter a 9 inch deep dish pie plate with the cold butter. In a food processor, pulverize the Oreos into crumbs. Pour crumbs into a mixing bowl and add the melted butter, stirring until fully combined. Pour the crumbs and butter mixture into the pie plate and press to evenly spread the crumbs across the bottom of the plate and up the sides. Press the crumbs firmly in place. Bake at 350 degrees for 10 minutes. Remove from the oven, let cool and refrigerate.
To make the mousse: In a mixing bowl, cream the butter and sugar with an electric mixer until light and fluffy. Add the eggs and mix until just blended. Add in the cream, cooled melted chocolate, and vanilla and mix until smooth. Pour into the chilled pie crust and then chill for one hour.
To make the espresso cream: In a small bowl, whisk together the sugar and corn starch and set aside. In a second small bowl, whisk together the egg yolks and ¼ cup of the milk. Set aside. In a third small bowl, combine the gelatin and water. (Stir if necessary so that the gelatin fully dissolves.) Set aside.
In a small saucepan, combine the remaining 1 ¼ cups milk and the espresso. Heat over medium high heat just until small bubbles start to form around the edges of the milk—but do not allow the milk to boil. If you are using espresso beans, strain the mixture (it’s ok if some of the espresso bean grinds remain but you want to strain out most of them). If you are using the instant espresso, no straining is necessary.
Whisk the sugar and cornstarch mixture into the hot milk mixture. Add a few tablespoons of the milk mixture at a time into the egg mixture to warm it up (this is called tempering and it will prevent the eggs from cooking and turning into scrambled eggs!); then add the egg mixture to the rest of the milk mixture in the pan. Cook the milk-egg mixture on medium heat until it thickens up slightly (about 2-3 minutes). Add the gelatin mixture and stir well until the gelatin is fully dissolved.
Remove the pan from the heat and pour into a bowl. Place this bowl over a larger bowl of ice water (called an ice bath) and stir constantly to cool and thicken some more (do not allow any of the ice water to get into the milk mixture). Keep stirring every few minutes to keep the mixture smooth.
In a large bowl, whip the heavy cream until stiff peaks form (being careful not to over whip the cream). Fold the espresso cream and whipped cream together until incorporated. Refrigerate about 30 minutes until cold.
Pour the espresso cream mixture over the mousse in the prepared pie crust and chill for 2 hours to set up before serving. (May be refrigerated overnight as well.)
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