Crust:
3 1/2 ounces semisweet chocolate, grated (reserve 1/2 ounce)
1/4 cup butter
1/2 cup all-purpose flour
1/8 teaspoon baking powder
2 large eggs
1 cup packed light brown sugar
1 1/2 teaspoons chocolate extract

Filling:
12 ounces cream cheese, room temperature
1 cup packed light brown sugar
3 large eggs
1/2 cup sour cream
1 1/3 cups creamy peanut butter

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

Preheat oven to 350 degrees F. Lightly grease & flour a 9- or 10-inch springform pan.

Crust:
In the top of a double boiler, over simmering water, melt 3 ounces of the chocolate & the butter, stirring until smooth. Remove from heat.

Combine the flour & baking powder. In a medium bowl, with electric mixer on medium, beat the eggs until thick & light-colored. Beat in the brown sugar until well blended. Beat in the melted chocolate, chocolate extract, & remaining 1/2 ounce of grated chocolate. Gradually stir in the dry ingredients, mixing just until blended. Spread 1 cup evenly onto bottom of the prepared pan. Chill the remaining crust mixture. Bake for 15-17 minutes, or until firm. Cool in the pan in the refrigerator for 15-30 minutes.

Filling:
In a large bowl, with electric mixer at medium, beat the cream cheese & brown sugar until smooth. Beat in the eggs & sour cream. Beat in the peanut butter. To prepare the pan, use a spatula to spread the remaining chilled crust evenly around the insides of the pan. To make this easier, set the pan on its side & roll it, spreading at the same time. Pour in the filling mixture. Bake for 25 to
30 minutes, or until the filling is firm, spreading the topping on the cheesecake about 3 minutes before removing from the oven.

Topping:
In a small bowl, with electric mixer on medium, mix the sour cream, sugar, & peanut butter until smooth.
Spread evenly over the top of the cake in the oven 2-3 minutes before it is removed. Cool on a wire rack for 1 hour. Place in a plastic or paper bag & chill overnight. Remove the side of the pan & garnish if desired.