Preheat oven to 350 degrees (176 celsius).
Lightly grease a 13x9x2-inch baking pan; set aside.
Cream together 3/4 cup butter and both sugars in a large bowl until light and fluffy.
Beat in eggs and vanilla.
In a seperate medium size bowl, combine 2 1/4 cups flour, baking powder and salt; blend into creamed mixture.
Divide batter in half.
Blend together cocoa powder and melted butter; stir into one half of the dough.
Spread cocoa dough evenly into the prepared pan.
Stir remaining 1/4 flour and 1/2 cup chocolate chips into remaining dough; spread evenly over the cocoa dough in the pan.
Optional, top with additional chocolate chips.
Bake 25 to 30 minutes or until edges start to pull away from sides of the pan.
Cut into bars.
Store in tightly covered container.