Heat oven to 350 degrees. Stir peanut butter until evenly mixed throughout; measure after stirring.
Combine brown sugar, 1-1/4 cups peanut butter and 1/2 cup butter in medium bowl; mix until smooth. Stir in eggs; beat until well mixed.
Combine flour, baking powder and salt; stir into peanut butter mixture. Stir until well mixed. If desired, stir in 1 cup peanut butter morsels. Pour into greased 13x9-inch baking pan.
Bake at 350 degrees for 25 to 30 minutes or until brownies begin to pull away from sides of pan. Cool on wire rack. Makes 2 dozen brownies.
For frosting: Combine remaining peanut butter, remaining butter, powdered sugar, vanilla and 3 tablespoons milk. Beat until smooth. Stir in remaining milk until frosting is spreading consistency. Spread over brownies.