Melt chocolate with 3 tablespoons of the butter over very low heat. Stir constantly until smooth. Set aside to cool.
Cream remaining 2 tablespoons butter with cream cheese until smooth. Gradually add 1/4 cup sugar. Cream until light and fluffy. Blend into this 1 of the eggs, 1 tablespoon flour and 1/2 teaspoon vanilla. Set aside.
Now beat the remaining 2 eggs until light and fluffy. Gradually beat into them 3/4 cup sugar. Continue beating until thickened. Stir in the baking powder, salt and 1/2 cup flour. Add to this the cooled chocolate mixture. Blend well. Stir in the nuts and 1 teaspoon vanilla.
Spread half of the chocolate batter into an 8x8 inch greased baking pan. Spread the cream cheese mixture over the top. Then drop the remaining chocolate batter by tablespoons over the top of the cream cheese mixture. Swirl through batter layers with a spatula for a marbled effect.
Bake in a preheated 350 degrees F (175 degrees C) oven for 35 to 40 minutes. Cool in the pan. Cut into squares or bars.