1. Prepare Brownie Crust: Preheat oven to 350°. Microwave first 2 ingredients in a small microwave-safe bowl at MEDIUM (50% power) 1 1/2 minutes or until melted, stirring at 30-second intervals. Stir together flour, salt, and baking powder in a bowl.
2. Beat 2 eggs and 1 cup brown sugar at medium-high speed with an electric mixer 3 to 4 minutes or until batter forms thin ribbons when beaters are lifted. Add vanilla, bittersweet chocolate, and melted chocolate mixture, and beat just until blended. Stir in flour mixture just until combined.
3. Spread 1 cup brownie mixture on bottom of a greased and floured 9-inch springform pan. Bake at 350° on center oven rack 13 to 15 minutes or until set. Cool on a wire rack 10 minutes; freeze 15 minutes. Remove from freezer; spread remaining brownie batter up sides of pan to 1/4 inch from top, sealing batter to bottom crust.
4. Prepare Filling: Beat cream cheese and 1 cup brown sugar at medium speed with a heavy-duty electric stand mixer until blended. Add 3 eggs, 1 at a time, beating just until yellow disappears after each addition. Beat in sour cream just until blended. Beat in peanut butter until blended.
5. Pour filling into prepared crust. (Mixture will not completely fill crust.) Bake at 350° for 35 minutes or until center is almost set.
6. Remove from oven. Spread Sour Cream Topping over center of cheesecake, leaving a 2-inch border around edge. Bake at 350° for 1 more minute. Remove from oven; gently run a knife around edge of cheesecake to loosen. Cool completely on a wire rack.
7. Cover and chill 8 to 12 hours. Remove sides of pan. Top with chocolate curls.
Note: We tested with Jif peanut butter.