In a large bowl, cream the shortening, peanut butter and brown sugar until light and fluffy. Beat in eggs and vanilla. Combine the flour, baking powder and salt; gradually add to creamed mixture alternately with milk, beating well after each addition.
Transfer to a greased 15-in. x 10-in. x 1-in. baking pan. Bake at 350° for 16-20 minutes or until a toothpick inserted near the center comes out clean. Cool on a wire rack.
For frosting, in a small bowl, cream the peanut butter, shortening and confectioners' sugar until light and fluffy. Gradually beat in enough milk to achieve spreading consistency. Frost bars.
In a microwave, melt chocolate chips and shortening; stir until smooth. Drizzle over frosting. Store in the refrigerator. Yield: 5 dozen.