Preheat oven to 350°.
In a mixing bowl, combine butter and peanut butter until smooth.
Add the granulated and brown sugars, mixing well.
Beat in the eggs, one at a time, until well blended. Add vanilla.
Slowly incorporate the flour, and baking powder. Stir in chocolate chips.
Pour into a greased 8" square pan and bake at 350° for 30-40 minutes or until the top is golden brown and a wooden pick inserted in to the center comes out clean. Let cook completely and cut into bars.