Crust
1 cup flour
1 cup finely chopped pecans
1/2 cup butter, softened
1/2 cup firmly packed brown sugar

Preheat oven to 325 degrees F.

Combine flour, pecans, butter and brown sugar; mix well. Press mixture onto the bottom of an ungreased 8- or 9-inch square baking pan. Bake 15 to 16 minutes, or until light golden brown. Let cool while preparing batter.

Batter
2 ounces (1/2 baking bar) bittersweet
    chocolate, broken into 1-inch pieces
24 ounces cream cheese, softened
3/4 cup granulated sugar
1 teaspoon pure vanilla extract
3 eggs

Melt chocolate in a double boiler over hot, but not boiling, water. Stir chocolate occasionally until smooth; set aside.

In a large mixing bowl, beat cream cheese, granulated sugar and vanilla extract until well blended. Add eggs, one at a time, mixing well after each addition. Remove 1 cup batter and reserve. Add melted chocolate to the mixing bowl and combine. Spoon chocolate batter over the crust. Pour the 1 cup of reserved plain batter over the chocolate batter. Using the blade of a butter knife, swirl the batter to create a marbled effect. Bake 30 to 35 minutes, or until center is set. Place the pan on a wire rack and cool to room temperature. Cover, and transfer the pan to the refrigerator to chill, approximately 3 hours. Store covered in the refrigerator.