In a large bowl, combine the sugar, the cocoa powder, the salt, and the flour, and mix well.

Add the applesauce, the eggs and the vanilla and beat until smooth.

Coat an 8x8 baking pan with non-stick cooking spray, and add half of the batter, spreading to fill the entire bottom.

In a separate bowl, combine all of the peanut butter filling ingredients, and beat until smooth.

Add the peanut butter filling by the tablespoon to the baking dish on top of the first brownie batter layer.

Add the other half of the original batter, and using a fork or a knife, swirl the top layer and the middle layer (this will give it a marbled appearance).

Bake for 30 minutes at 325 degrees Fahrenheit and let cool.

In a small bowl, combine all of the ingredients for the frosting and stir until well combined.

Spread over the brownies, cut and serve.