1. Lightly grease 9×13 inch baking pan.
2. Preheat oven to 350 degrees F.
3. In a medium bowl, beat together first 5 ingredients until smooth.  Set aside.
4. In a large bowl, mix together melted butter, 2 cups sugar, and vanilla.  Mix in eggs, one at a time.
5. Combine flour, cocoa, baking powder, and salt; mix into the batter.
6. Stir in chocolate chips.  Spread batter into prepared pan.
7. Drop the peanut butter filling on top by the large spoonful.
8. Using a butter knife, gently swirl peanut butter filling through the chocolate batter to create a marbled effect. (The batter is stiff so this is a little difficult. The key is not to over swirl it.)
9. Once you are done swirling, you may need to tap the pan on a towel covered counter to make the batter settle.
10. Bake in preheated oven for 35-40 minutes or until a toothpick inserted one inch from the edge comes out clean.
11. Cool completely, then cut into small squares.

Notes:

1. *To cut calories you can substitute 1/4 cup butter for 1/4 cup of applesauce.  In fact, with most recipes you can substitute up to 1/3 of the fat with applesauce without compromising the taste.