Preheat oven to 325 degrees.

Grease and flour a 14 by 12-inch sheet cake pan.

Melt chocolate, 6 tablespoons butter, and margarine in top half of a double boiler over boiling water, stirring to blend. When melted, remove from heat and allow to cool.

Sift together flour, baking powder, baking soda, and salt and set aside. In the mixing bowl, beat 2 eggs and vanilla until frothy. Add sugar and continue beating until thick and lemon-colored. Gradually beat in flour mixture until well combined. Stir in chocolate mixture, scraping down sides of bowl as necessary. When well blended, pour into prepared pan. Smooth top and sprinkle with 1/2 cup walnuts. Set aside.

Beat cream cheese and remaining 4 tablespoons butter until light and fluffy. Add remaining egg and beat to combine. Add confectioners sugar and beat until well blended.

Spread cream cheese mixture over top of brownies, leaving 1/2-inch border all around, using a rubber spatula to smooth the surface. Sprinkle with remaining 1/2 cup walnuts.

Place in preheated oven and bake for 30 to 40 minutes or until set.

Remove from oven and cool on a wire rack for 2 hours before cutting into squares.