Preheat oven to 350 degrees. In a large bowl, cream the butter, peanut butter, and both sugars. Add eggs in one at a time beating after each one. Add vanilla and stir. In a separate bowl, combine flour, baking powder, and salt. Gradually add flour mixture to egg mixture. Grease an 8 x 8 or 9 x 9 pan. Spread 1/2 of mixture in bottom of pan. Unwrap chocolate bars and layer them over mixture (as shown in picture below). Spread remaining mixture on top of chocolate. Bake at 350 degrees for 30 - 35 minutes or until a toothpick inserted into the center comes out clean. Cool then cut into bars. Serve slightly warm for the best results.