Preheat oven to 325°F (163°C). Grease a 9-inch springform pan and line the bottom with parchment paper.
In a bowl, mix chocolate cookie crumbs, melted butter, and sugar until combined. Press into the bottom of the springform pan. Bake for 8 minutes, then let cool.
In a large bowl, beat cream cheese, sugar, and cocoa powder until smooth. Add eggs one at a time, mixing after each addition. Stir in Irish cream, sour cream, vanilla, and salt until fully incorporated. Fold in melted chocolate until smooth.
Pour the batter over the crust. Wrap the outside of the pan with foil and place it in a water bath (a larger pan with hot water about 1 inch deep).
Bake for 55-60 minutes, or until the center is slightly set but still jiggly. Turn off the oven and crack the door open slightly. Let the cheesecake cool in the oven for 1 hour before transferring to the fridge. Chill for at least 4 hours or overnight.
Heat heavy cream in a saucepan over medium heat until steaming (but not boiling). Pour over chopped dark chocolate, let sit for 2 minutes, then stir until smooth. Mix in 1 tablespoon Irish cream liqueur for extra flavor.
Pour the ganache over the chilled cheesecake, spreading it evenly. Let it set for 15 minutes before slicing. Garnish with whipped cream, chocolate shavings, or crushed cookies.