Preheat oven to 350°F Lightly grease 13 x 9 inch baking pan. In saucepan, melt chocolate chips and butter over medium heat, stirring constantly. Set aside to cool.
While mixture cools, make topping: Cream butter and cream cheese together then add sugar, eggs, and flour. Beat until fluffy and set aside.
When chocolate mixture has cooled to the touch, wait about 10-15 minutes, then finish brownie base. Beat eggs, add sugar and mix. Then, mix in flour, baking powder and vanilla. Add chocolate mixture and stir, but do not overheat. Pour batter into prepared pan. Pour cream cheese mixture over batter. Swirl with knife edge to make marbled effect. Sprinkle pecans over top. Bake 45 to 50 minutes. When cool, cut into squares and serve.