Brownie Crust
3 ounces unsweetened chocolate, chopped
1/4 cup (1/2 stick) unsalted butter, cut into small pieces
1/2 cup sifted all purpose flour
1/8 teaspoon salt
1/8 teaspoon baking powder
2 large eggs
1 cup firmly packed golden brown sugar
1 1/2 teaspoons vanilla extract
1/2 ounce bittersweet (not unsweetened) or
    semisweet chocolate, finely chopped

Position rack in center of oven and preheat to 350 degrees F. Butter a 9-inch diameter springform pan with 2 3/4-inch-high sides. Dust pan lightly with flour, tapping out excess.

Melt unsweetened chocolate and butter in heavy small saucepan over low heat. Cool.

Blend flour, salt and baking powder in small bowl. Using electric mixer, beat eggs and sugar in medium bowl until slowly dissolving ribbon forms when beaters are lifted, about 4 minutes. Scrape down sides of bowl. Beat in melted chocolate mixture, vanilla extract and chopped chocolate. Add dry ingredients and mix until just blended; do not over-beat.

Spread 1 cup brownie batter over bottom of prepared pan. Bake until firm, about 17 minutes. Cool baked bottom crust in pan in freezer, about 15 minutes. Maintain oven temperature.

Meanwhile, prepare Filling.

Filling
12 ounces cream cheese (at room temperature)
1 cup firmly packed golden brown sugar
3 large eggs
1/2 cup sour cream
1 1/3 cups (about 12 ounce) creamy peanut butter
    (do not use old-fashioned or freshly-ground)

Blend cream cheese and sugar in processor until smooth. Add eggs and sour cream and process using on/off turns until mixture is smooth, stopping to scrape down sides of bowl as needed. Blend in peanut butter using on/off turns.

Using small knife, spread remaining brownie batter evenly around sides of pan, sealing batter to bottom crust. Pour in filling, which will not be as high as brownie batter. Bake until center of cheesecake is gently set and brownie sides have fallen to form a 1-inch-wide ring around filling, about 30 minutes.

Topping
3/4 cup sour cream
2 teaspoons granulated sugar
1/2 cup creamy peanut butter

Blend sour cream and sugar in small bowl. Spread evenly atop cheesecake to within 3/4 inch of edge. Bake cheesecake 1 more minute.

Set cake on rack. Using small sharp knife, cut around top edge of cake to loosen from pan sides. Cool completely. Place peanut butter in pastry bag fitted with small (no. 1) star tip. Pipe tiny rosettes of peanut butter around edge of sour cream. Refrigerate cake until rosettes are firm, about 30 minutes. Cover pan tightly with foil and chill overnight. (Can be prepared 1 day ahead.)

Cut around pan sides to loosen cake. Release sides. Transfer cheesecake to platter. Cut into wedges and serve.

Yields 12 servings.