CRUST
Add the Oreos to a food processor and pulse until very fine crumbs form.
Transfer the crumbs to a mixing bowl. Pour in the melted butter and mix until the texture feels like wet sand.
Cover the bowl and set aside.
CHOCOLATE MOUSSE
Place the chopped chocolate in a medium heat-safe bowl.
In a small saucepan over medium-low heat, add the heavy cream, butter, and sugar. Whisk until the mixture is hot and the sugar is dissolved, but do not let it boil.
Pour the hot cream mixture over the chocolate, cover the bowl, and let it sit undisturbed for 10 minutes.
Uncover and whisk from the center outward until completely smooth and glossy. Let it cool slightly so it’s warm but not hot.
Whisk in the egg yolks until fully combined.
Add the espresso powder, vanilla extract, and a tiny pinch of salt, then whisk again until smooth.
In a separate bowl, whip the chilled heavy cream with a hand mixer or whisk until soft peaks form.
Add the whipped cream to the chocolate mixture in batches, gently folding after each addition until the mousse is light and airy.
Cover the mousse, trasnfer to the fridge, and set aside.
CHANTILLY CREAM
In a clean bowl, add the heavy cream, powdered sugar, and vanilla extract.
Whip with a hand mixer or whisk until soft peaks form.
ASSEMBLE
Add a few spoonfuls of the crust mixture to the bottom of each cup and press down lightly to form an even layer.
Spoon or pipe the chocolate mousse on top so each cup fills about two-thirds to three-quarters of the way up.
Finish the remaining space with Chantilly or whipped cream.
Enjoy right away, or cover each jar and refrigerate for up to 3 days.