Preheat the oven to 325°F / 165°C and mix the chocolate cookie crumbs with melted butter, then press into the bottom of a 9-inch springform pan.
Beat the cream cheese and sugar until smooth, add the eggs one at a time, then fold in the melted dark chocolate and cocoa powder until fully combined.
Pour the chocolate cheesecake batter over the crust and bake for 50 to 55 minutes until the edges are set but the center still jiggles slightly.
Cool the cheesecake to room temperature, then refrigerate for at least 4 hours or overnight until fully chilled.
Spread the whipped cream evenly over the chilled cheesecake to create the mousse layer, then dust with cocoa powder before serving.