Grease a 9-inch springform pan with butter.

Mix crushed Oreo cookies with melted butter until combined.

Press firmly into the base of the pan and chill for 30 minutes.

In a bowl, combine chopped chocolate, vanilla, and salt.

Heat 3/4 cup heavy cream until just simmering, then pour over chocolate.

Let sit for 1 minute, then whisk until smooth. Chill for 20 minutes.

Whip 1 1/2 cups heavy cream to soft peaks.

Gently fold the cooled chocolate mixture into the whipped cream until smooth.

Spread the mousse over the chilled crust and smooth the top.

Refrigerate for at least 6 hours or overnight.

Whip remaining cream with sugar to stiff peaks.

Pipe whipped cream on top and garnish with Oreo halves and chocolate shavings.

This Chocolate Mousse Cake should always be served chilled for the best texture and flavor.