To make crust:
Preheat oven to 350°F. Lightly butter a 9-inch round spring form pan.
Combine cookie crumbs and butter until thoroughly combined. Transfer mixture to prepared pan and press evenly and firmly into bottom of pan. Bake for 15 minutes. Set aside to cool slightly.
To make cheesecake:
Using electric mixer on medium speed, beat cream cheese and sugar until thoroughly combined and smooth. Add eggs, liqueur and vanilla and mix well. Mix in chocolate. Mix in sour cream.
Transfer cheesecake batter to pan and spread evenly. Bake for 15 minutes at 350°F.
Reduce oven temperature to 200°F. Bake cheesecake for 2 hours.
Turn off oven. Remove cheesecake from oven and gently run a thin knife around the edge of the pan.
Return cheesecake to oven and leave it inside oven with heat off for 2 hours. This will allow the cheesecake to cool to room temperature without cracking.
Remove cheesecake from oven. Cover loosely and chill overnight.
To make whipped cream:
Using electric mixer with whisk attachment, beat cream at medium-high speed until the cream begins to thicken. Sprinkle sugar over cream and continue beating until soft peaks form.
Before serving the cheesecake, spoon whipped cream into pastry bag and pipe whipped cream around edge of cheesecake. Sprinkle with cocoa powder.