Preheat oven to 350 degrees F. Grease a 13x9-inch pan; set aside.
In a medium bowl, whisk flour, cocoa powder, baking powder and salt then; set aside.
In a large bowl fit with a mixer, combine 2 cups sugar and 1 cup softened butter: beat until light and fluffy. Add 4 eggs, 1 at a time, beating well after each addition. Stir in vanilla extract.
Gradually add flour mixture to sugar/butter mixture; blend well. Stir in peanut butter chips; set aside.
To make the peanut butter filling - In a small bowl, combine both peanut butters and 1/3 cup butter; beat until smooth. Add 1/3 cup sugar and 2 tablespoons flour; blend well. Add 3/4 cup teaspoon vanilla extract and 2 eggs; beat until smooth.
To assemble, spread half brownie batter in greased pan. Scoop peanut butter filling on top and smooth with spatula. Spread remaining brownie batter on top.
Using a knife to marble the batter, pull knife through layers in wide curves.
Bake at 350 degrees F for 40-50 minutes or until top springs back when touched or toothpick inserted into the center comes out clean. Cool completely.
To make the frosting - In a medium saucepan over low heat, melt chopped chocolate and 3 tablespoons butter, stirring until smooth. Remove from heat and stir in Reese's peanut butter spread until melted. Stir in powdered sugar, salt, vanilla extract and enough water for desired spreading consistency.
Frost brownies and cut into bars.