Grease a 9-inch springform pan with butter.

In a bowl, mix the finely crushed Oreo cookies with the melted butter until evenly coated.

Press the mixture firmly into the bottom of the springform pan using the bottom of a flat measuring cup. Chill in the refrigerator for 30 minutes.

Meanwhile, place the chopped semisweet chocolate, vanilla extract, and salt in a medium bowl.

In a small saucepan, heat 3/4 cup of the heavy cream until it reaches a bare simmer.

Pour the hot cream over the chocolate mixture. Cover and let it sit for 1 minute, then whisk until smooth.

Cover and chill the chocolate ganache in the refrigerator for about 20 minutes, or until cooled.

In a separate medium bowl, beat the remaining 1 1/2 cups heavy cream with a hand mixer or stand mixer on medium-high speed until soft peaks form, about 2 minutes.

Whisk the cooled chocolate mixture to loosen it, then gently fold it into the whipped cream with a silicone spatula until fully combined and no streaks remain.

Spoon the Chocolate Mousse Cake filling over the chilled crust and gently spread it evenly to the edges.

Cover and refrigerate for at least 6 hours or overnight.

Just before serving, beat the remaining 3/4 cup heavy cream with the sugar until stiff peaks form.

Pipe the whipped cream around the border of the cake using a large open star tip.

Garnish with Oreo cookie halves and chocolate shavings.

Keep refrigerated until ready to serve.