Preheat oven to 350°F. Grease a 9-inch springform pan.
Pour Guinness in a small saucepan and place on low-medium heat. Let simmer until mixture reduces to 3/4 cup. This may take up to 30 minutes, so make this the first thing you start. Let mixture cool.
In a medium-sized bowl combine all ingredients for the crust. Press on the bottom and up sides of pan. Bake for 10 minutes and let cool.
Place cream cheese and sugar in a large bowl. Using an electric mixer, beat together cream cheese and sugar until light and creamy.
Add eggs, one at a time. Mix in sour cream and vanilla extract.
Place bittersweet chocolate in a microwavable bowl. Place on high heat for 90 seconds, stopping to stir every 15 seconds. Once chocolate is completely melted and smooth remove from microwave and let cool.
Mix melted chocolate into cream cheese mixture. Mix in cooled Guinness reduction.
Mix in flour and cocoa powder.
Reduce oven temperature to 300°F. Pour cream cheese mixture into crust. Prepare cheesecake pan for a water bath. If not using a water bath, place a pie pan filled with water on the lower rack of the oven. Bake 1 hour and 40 minutes. Turn off oven. Leave cheesecake in oven for 30 minutes with door closed. Open oven door and leave cheesecake in oven for another 30 minutes. Place cheesecake in refrigerator and let cool at least 4 hours, preferably overnight.