Melt 9 ounces semisweet chocolate and 1 cup unsalted butter in the microwave for about 1 minute (20 seconds at a time and stirring well before microwaving it again) or in a double boiler over, but not touching, the water.
Add 1 ½ cups sugar and mix well. Add 6 eggs, two at a time, and whisk, making sure it's all integrated well. Don't beat.
Add the chocolate mixture, 2 tablespoons unsweetened cocoa powder, 1 teaspoon vanilla extract and mix until smooth.
Stir in 1 cup all-purpose flour and ¼ teaspoon salt. Mix only until combined.
Pour the chocolate mixture in the prepared pan. Drizzle cheesecake batter unevenly over it. With the tip of a knife, swirl lightly, trying to bring some of the dark batter to the top. Stop before you have too many swirls and the cheesecake part is too mixed.
Bake for about 25 minutes, until the top dries but the center is still jiggly.
Remove, let cool on a wire rack, cover with plastic wrap and refrigerate until firm to cut. I like to eat them the next day.
Store leftovers covered in the refrigerator or freeze for up to a month.