• Preheat the oven to 300°F.
• Grease a 9-inch springform pan and then line with parchment paper (the bottom and sides). Grease the paper as well, then dust with cocoa powder.

Make the crust:

• In the top of a double boiler over lightly simmering water, melt chocolate and butter. Remove from heat, and stir in sugar, cocoa powder, eggs, and vanilla. Pour into prepared pan.
  4 ounces semi-sweet chocolate, ½ cup butter, ¾ cup sugar, ½ cup cocoa powder, 3 eggs, 1 teaspoon vanilla extract
• Bake for 30 minutes. Remove from oven and use the back of a spoon to smush the cake down to make a thinner crust. Let cool to room temperature before adding the cheesecake mixture.

Make the cheesecake:

• Preheat the oven to 325°F.
• In a large mixer bowl, beat cream cheese and sugar until light and fluffy. Add eggs one at a time while beating. Stir in sour cream, vanilla, and Bailey's or coffee. Stir in the melted chocolate.
  16 ounces cream cheese, ½ cup sugar, 2 eggs, 1⅓ cups sour cream, ¾ teaspoon vanilla extract, ¼ cup Bailey's Irish Cream or strong chilled coffee, 6 ounces semi-sweet chocolate
• Carefully wrap bottom and sides of the springform in aluminum foil and place pan in a larger baking pan filled halfway with water. This water bath step helps prevent cracks in the cheesecake.
• Pour the cheesecake mixture over the crust. Place the pan on the lower middle rack and bake the cheesecake for approximately 50 minutes or until it no longer jiggles in the center.
• Turn the oven off and prop the oven door open with a wooden spoon. Keep the cheesecake in place for 1 hour.
• Let cool to room temperature. Loosen the sides of the pan from the cheesecake by running a small icing spatula or knife around the perimeter. Proceed to mousse step or cover cake with aluminum foil and refrigerate until the next day.

Make the dark chocolate mousse:

• Place the chocolate in a large bowl set over low simmering water (or in the top of a double boiler). Stir chocolate until melted. Turn off the heat and let stand to reach room temperature.
  5¼ ounces bittersweet chocolate
• Beat the cream over ice until it forms soft peaks. Set aside and hold at room temperature.
  14 ounces cold heavy cream
• With a mixer, whip egg to soft peaks. Gradually add the sugar and continue whipping until firm.
  3 large egg whites, 1 ounce sugar
• Transfer the chocolate to a large bowl. Using a spatula, fold the egg whites into the chocolate. When the eggs whites are almost completely incorporated, fold in the whipped cream.
• Place an acetate collar around the perimeter of the pan, if desired.
  
• Add mousse on top of cheesecake and smooth top. Cover cake with foil and refrigerate until ready to top with ganache.

Make the ganache:

• Melt the chocolate in the top of a double boiler, in a bowl on top of simmering water, or in the microwave at 50% power in 30-second intervals.
  4 ounces bittersweet chocolate
• Boil the cream, then pour over the melted chocolate. Whisk until smooth. It will come together, just keep whisking. Add the corn syrup and mix completely.
  ½ cup heavy whipping cream, 2 teaspoons light corn syrup
• Allow to cool briefly. Do not allow the ganache cool too much or it will start to harden when it hits the cold mousse and be harder to spread. If you'd like the ganache to drip down the sides of the cheesecake, remove the acetate collar, if using, and the springform pan sides. Place the cake on a platter lined with wax paper (the wax paper just needs to cover the bottom edges of the cake).
• Spread the ganache over mousse layer, allowing to drip down sides if desired. Sprinkle with cocoa powder if desired. Cover cake with aluminum foil and chill.