Preheat the oven to 350°F (177°C). In a food processor or blender, pulse 22 Oreos (including the cream filling) into fine crumbs. Pour the crumbs into a medium bowl. Add the melted butter and stir to combine. The mixture will be thick and quite wet. Try to smash/break up any large chunks. Pour the mixture into an ungreased 9-inch pie dish. With medium pressure using your hand, pat the crumbs down into the bottom and up the sides to make a compact, thick crust. Bake for 10 minutes, and then place on a cooling rack to cool completely.

Microwave the chopped chocolate and butter together in a large heat-proof bowl, stopping and stirring every 20 seconds until completely melted. Set aside.

Place the egg whites, sugar, and cream of tartar in a heat-proof bowl or the upper pot of a double boiler. Set bowl over a saucepan filled with 2 inches of simmering water. Do not let the bowl touch the water. Whisking constantly, cook the egg white mixture until the sugar has dissolved and the mixture reaches 160°F (71°C) on an instant-read thermometer.

Transfer the mixture to a heat-safe bowl (if using a hand mixer) or the bowl of a stand mixer fitted with a whisk attachment. Beat on high speed until soft, glossy peaks form, about 5–6 minutes. See photo above for a visual. Gently fold the melted chocolate into the egg white mixture until combined. Wash the mixer bowl and whisk attachment clean.

Using a handheld or stand mixer fitted with a whisk attachment, whip the heavy cream on medium-high speed until medium-to-stiff peaks form, about 3–4 minutes. Gently fold whipped cream into the chocolate mixture. It may seem like a lot of whipped cream, but just keep folding and it will eventually come together. Do not over-mix; it will deflate the mousse. (No need to wash or wipe the bowl/whisk attachment clean—you’ll make sweetened whipped cream in step 7.)

Using a small offset spatula, spread half of the mousse in the cooled pie crust. Sprinkle the crushed cookie crumbs evenly over the top. Top with the remaining chocolate mousse, mounding it in the center to create a dome shape. Place the pie in the refrigerator while you prepare the sweetened whipped cream topping.

Using a handheld or stand mixer fitted with a whisk attachment, whip the heavy cream, sugar, and vanilla on medium-high speed until medium peaks form, about 3 minutes. You want the peaks to be somewhat soft so the whipped cream spreads nicely on the mousse.

Remove the pie from the refrigerator and spread the whipped cream on top—keeping that dome shape in the center. Top with chocolate shavings, if using. Refrigerate the pie, uncovered, for at least 4 hours or up to 1 day. Let it sit at room temperature for 10 minutes before slicing and serving. Store leftover pie, covered, in the refrigerator for up to 4 days.