Marbled Brownies

INGREDIENTS

Makes 24
8 ounces plain chocolate
3 ounces butter
4 eggs
10½  ounces sugar
5 ounces all-purpose flour
½ teaspoon salt
1 teaspoon baking powder
2 teaspoons vanilla extract
4 ounces walnuts, chopped
For the plain mixture
2 ounces butter, at room temperature
6 ounces cream cheese
3½ ounces sugar
2 eggs 
1 ounces all-purpose flour
1 teaspoon vanilla extract
Preheat  a 350°F oven. Line a 13 x 9 inch baking pan with waxed paper and grease.
Meanwhile, beat the eggs until light and fluffy. Gradually add the sugar and continue beating until blended. Sift over the flour, salt and baking powder and fold to combine.
Melt the chocolate and butter over very low heat, stirring constantly. Set aside to cool.
Stir in the cooled chocolate mixture. Add the vanilla and walnuts. Measure and set aside 16 fluid ounces of the chocolate mixture.
For the palin mixture, cream the butter and cream cheese with an electric mixer.
Add the sugar and continue beating until blended. Beat in the eggs, flour and vanilla extract.
Spread the unmeasured chocolate mixture in the tin. Pour over the plain mixture. Drop spoonfuls of the  reserved chocolate mixture on top.
With a metal spatula, swirl the mixtures to marble. Do not blend completely. Bake until  just set, 35-40 minutes. Turn out when cool and cut into squares for serving.
Source: (The Around the World Cookbook)