Microwave first 3 ingredients in a 1-quart glass bowl at HIGH 2 minutes or until melted, stirring once. Cool and set aside.
Beat cream cheese and 1/4 cup butter at medium speed with an electric mixer until creamy; gradually add 1/2 cup sugar, beating well.
Add 2 eggs, 1 at a time, beating until blended. Stir in 1 teaspoon vanilla. Fold in 2 tablespoons flour and 1/2 cup chocolate morsels; set aside.
Beat remaining 4 eggs in a large bowl at medium speed with an electric mixer. Gradually add remaining 1 1/2 cups sugar, beating well. Add melted chocolate mixture and 2 teaspoons vanilla extract; beat mixture until well blended.
Combine 1 cup flour, baking powder, and salt; fold into chocolate batter until blended, and stir in remaining 1 cup chocolate morsels.
Reserve 3 cups chocolate batter; spread remaining batter evenly in a greased 13- x 9-inch pan. Pour cream cheese mixture over batter. Top with reserved 3 cups batter, and swirl mixture with a knife.
Bake at 325° for 40 to 45 minutes. Cool and cut brownies into squares.