Heat oven to 350. Greased 13 x 9 inch pan. In large bowl, beat 2 cups of sugar and 1 cup of butter until light and fluffy. Add 4 eggs, one at a time, beating well after each addition. Add 2 t. vanilla; blend well. In small bowl, combine 1 1/2 cups flour, cocoa, baking powder and 1/2 t. salt. Gradually add flour mixture to sugar mixture; mix well. Stir in peanut butter chips.
In small bowl, beat peanut butter and 1/3 cup butter until smooth. Add 1/3 cup sugar and 2 T. flour; mix well. Add 3/4 t. vanilla and 2 eggs; blend well. Spread half of chocolate mixture in greased pan. Spread peanut butter mixture evenly over chocolate mixture. Spread remaining chocolate mixture evenly over peanut butter mixture. To marble, pull knife through layers in wide curves.
Bake at 350 for 40-50 minutes or until top springs back when touched lightly in center and brownies begin to pull away from sides of pan. Cool completely.
In medium saucepan over low heat, melt chocolate and 3 T. butter, stirring constantly until smooth.
Remove from heat, stir in powdered sugar, 1/4 t. salt, 3/4 t. vanilla and enough water for desired spreading consistency. Frost cooled brownies; cut into bars.