Grease a 13" x 9" baking pan and preheat oven to 350 degrees.
In a 2-quart heavy saucepan over low heat, melt butter and chocolate.
With a wire whisk or wooden spoon, beat in 2 cups sugar and 4 eggs until well blended.
With wooden spoon, stir in flour, salt, nuts and 1 teaspoon vanilla extract.
Evenly spread batter in pan.
In small bowl with mixer at low speed, beat cream cheese, 1/2 cup sugar, 1 egg, 2 tablespoons flour and 1 teaspoon vanilla extract until just mixed; increase speed to medium and beat 2 minutes, occasionally scraping bowl. With large spoon, drop mixture in dollops on top of batter. Using tip of knife, lightly score top surface in a crisscross pattern.
Bake 40 to 45 minutes until toothpick inserted in center comes out clean.
Cool on wire rack. Cut into bars. Refrigerate bars up to 1 week.