Make sure the oven rack is in the center position and preheat the oven to 375 degrees F.
In a large mixing bowl, combine all the crust ingredients with a fork and mix until well combined. Press the cookie mixture into the bottom of a 9-inch springform pan. Bake the crust until fragrant and just set, about 8 to 10 minutes. Using oven mitts or pot holders, remove from the oven and cool for at least 15 minutes before adding the filling.
Lower the oven temperature to 350 degrees F.
Place semisweet chocolate in a microwave-proof glass bowl or measuring cup and cover it with plastic wrap. Microwave the chocolate on high power for 30 seconds; then stir it with a spoon or rubber spatula. Continue to microwave the chocolate on high, stopping to stir every 30 seconds, until it is completely melted and smooth. (If you don't have a microwave, you can use the double-boiler method for melting chocolate.) Set aside.
Place the cream cheese, sugar, and vanilla extract in a large metal bowl and, using an electric mixer, beat until creamy and smooth, about 4 minutes. Turn the mixer off and scrape down the sides of the bowl with a rubber spatula. Sift the cocoa powder into the cream cheese mixture and beat to incorporate. Turn the mixer off.
Crack and add the eggs 1 at a time, beating well after each addition. Be sure to turn the mixer off before you add each egg. *Add the sour cream and melted chocolate and beat until smooth. Using a rubber spatula, transfer the batter to the prepared pan with the baked crust.
Bake at 350 degrees F until the cake rises and puffs and the center is just set and still slightly wiggly, about 1 hour and 10 minutes. Using oven mitts or pot holders, remove from the oven and place on a wire rack to cool.
Grate the chocolate bar and sprinkle the shavings over the top of the cheesecake while it's still hot.
Let the cheesecake cool for 1 hour. Cover the cheesecake with plastic wrap and refrigerate at least 4 hours or overnight before serving.
When you're ready to serve the cheesecake, gently run a thin sharp knife around the inside edge of the springform pan to loosen the cake's sides, then unhook and carefully remove the outer ring of the pan. Slice and serve.
Be careful not to touch the hot pan when you add the grated chocolate to the hot cake! *
*It's important not to add hot melted chocolate to the raw eggs, or you'll cook them!
Kick up your cheesecake by topping it with some fresh whipped cream.