Combine finely crushed Oreo cookies and melted butter. Press into the bottom of the springform pan using the bottom of a flat measuring cup. Make sure it is compact. Chill for 30 minutes in the fridge.
Combine the chopped chocolate, vanilla extract, and the salt in a medium bowl.
In a small saucepan, bring 3/4 cup of the heavy cream to a bare simmer. Pour the cream over the chocolate, let sit for 1 minute, then whisk until smooth. Cover and chill in the fridge for about 20 minutes to cool.
Beat 1 1/2 cups heavy cream in a medium bowl with a hand mixer or stand mixer on medium-high speed to stiff peaks, about 2 minutes. Whisk the chocolate mixture to loosen it then fold it into the whipped cream with a silicone spatula until no streaks remain.
Scrape the chocolate mousse into the pan and gently spread it to the edges. Cover and refrigerate for at least 6 hours or overnight.
Just before serving, beat the remaining 3/4 cup of cream and 2 tbsp sugar with a hand mixer or stand mixer until the cream reaches stiff peaks. Pipe the borders of the cake using a large open star tip. Garnish with Oreo Cookie halves and chocolate shavings. This cake must stay refrigerated.
Recipe originally published June 2017. Updated with recipe video.