Preheat oven to 350 degrees F (175 degrees C ). Grease a 9x9 inch baking pan.
In a medium bowl, cream together peanut butter and margarine. Gradually blend in the brown sugar, white sugar, eggs, and vanilla; mix until fluffy. Combine flour, baking powder, and salt; stir into the peanut butter mixture until well blended.
Bake for 30 to 35 minutes in preheated oven, or until the top springs back when touched. Cool, and cut into 16 squares.