Preheat oven to 150 degrees C. Grease an 20cm square baking pan.
In a large mixing bowl, cream together the peanut butter and butter. Add the brown sugar, vanilla and eggs and beat until light and fluffy. Combine the flour, baking powder and salt in a separate bowl. Add to the butter mixture and mix until well blended. Stir in the peanuts.
Spread batter evenly into pan and bake for 25 to 30 minutes or until toothpick comes out clean. Let cool on wire rack and cut into 5cm squares.