Preheat the oven to 350 degrees. Spray a 9-inch springform pan with cooking spray and dust lightly with flour. Set aside.

Combine the unsweetened chocolate and butter in a microwave-safe bowl and heat in 30-second intervals, stirring after each interval, until mixture is melted and smooth. Cool for several minutes before proceeding.

Combine the flour, salt, and baking powder in a small bowl. Cream the eggs and sugar in a medium bowl with an electric mixer set to medium for 3 or 4 minutes, until the batter forms ribbons when the beaters are lifted. Beat in the melted chocolate, vanilla, and chopped semisweet chocolate. Add the dry ingredients and mix until just incorporated.

Pour 1 cup of the batter in the prepared pan and bake for 16 to 17 minutes, until firm to the touch. Cool for 10 minutes, then freeze for 15 minutes.

Spread the remaining brownie batter around the edge of the pan with a small offset spatula. (See photo at the bottom of post for a visual illustration.) Set aside while you make the filling.

Place the cream cheese and brown sugar in the bowl of a food processor and process until smooth. Scrape down the sides and bottom of the bowl, then add the eggs and sour cream and pulse until mixture is smooth. Pour into the prepared pan; the filling will not come up as high as the brownie batter. Bake for 30 minutes, until the center is almost set.

While cheesecake is in the oven, mix the sour cream and sugar in a small bowl. Spread over the top of the hot cheesecake to within 3/4 of an inch of the edge. Bake for 1 minute more. Immediately run a small knife between the cheesecake and the pan, then cool completely on a wire rack. Place the peanut butter in a pastry bag fitted with a small star tip and pipe stars around the perimeter of the cheesecake. Refrigerate for 30 minutes, until stars are firm. Cover with foil, then chill an additional 8 to 24 hours. Serve drizzled with chocolate sauce, if desired.