Preheat the oven to 350°F (177°C).

In a food processor, pulse 22 Oreos into fine crumbs.

Pour into a medium bowl and mix with the melted butter.

Press this mixture into the bottom and sides of a 9-inch pie dish.

Bake for 10 minutes, then let cool completely.

Microwave the chopped chocolate and 2 tablespoons of butter together in a heat-proof bowl, stirring every 20 seconds until melted. Set aside.

In another heat-proof bowl, combine egg whites, sugar, and cream of tartar.

Place the bowl over a saucepan of simmering water (but don’t let it touch the water).

Whisk constantly until the mixture reaches 160°F (71°C).

Transfer the mixture to a mixer and beat on high speed until soft peaks form, about 5–6 minutes.

Gently fold the melted chocolate into the egg whites.

In a separate bowl, whip 2 cups of heavy cream until medium-to-stiff peaks form.

Fold this into the chocolate mixture carefully.

Spoon half of the mousse into the cooled crust.

Sprinkle the crushed cookie crumbs on top, then layer the remaining mousse, forming a dome shape in the center.

Refrigerate the pie.

For the topping, whip 1 cup of heavy cream with sugar and vanilla until medium peaks form.

Spread this whipped cream on top of the mousse, keeping the dome shape.

Add chocolate shavings if desired.

Refrigerate uncovered for at least 4 hours or up to 1 day.

Allow the pie to sit at room temperature for 10 minutes before slicing and serving.

Store leftovers covered in the fridge for up to 4 days.