Preheat oven to 350. Line bottom and sides of a 13x9 baking dish with wax paper and grease.

Melt the chocolate and butter in a saucepan over low heat. Stir constantly until melted, Let cool.

Meanwhile, beat eggs until light and fluffy. Gradually add in sugar and continue beating until well blended. Stir in flour, baking powder and salt. Fold to combine. Stir in cooled chocolate. Add vanilla and nuts. Set aside.

For the plain batter, cream the butter and cream cheese with an electric mixer. Add sugar and continue beating until blended. Beat in eggs, vanilla, and flour. Spread half of the chocolate mixture in the bottom of the greased pan. Pour the cream cheese mixture over the top of the chocolate. Drop the remaining chocolate batter on top of the cream in large spoonfuls. With a butter knife or metal spatula swirl the mixtures to marble. Be careful not to over blend. Bake until just set, about 35-40 minutes. Let cool in pan before unmolding and cutting into squares. Makes about 24 squares.