1. Heat oven to 350°F. Grease 13x9x2-inch baking pan.
2. Stir together butter, sugar and 1 teaspoon vanilla in large bowl. Add 2 eggs; stir until blended. Stir together flour, cocoa, baking powder and salt. Add to egg mixture, stirring until blended. Spread in prepared pan. Bake 20 minutes.
3. Meanwhile, stir together sweetened condensed milk, peanut butter, remaining egg and remaining 1 teaspoon vanilla extract. Pour evenly over hot brownie. Set aside 1 tablespoon each milk chocolate chips and peanut butter chips; sprinkle remaining chips over peanut butter mixture. Return to oven; continue baking 20 to 25 minutes or until peanut butter layer is set and edges begin to brown. Cool completely in pan on wire rack.
4. Stir together remaining milk chocolate chips, remaining peanut butter chips and shortening in small microwave-safe bowl. Microwave at MEDIUM (50%) 30 seconds; stir. If necessay, microwave at MEDIUM an additional 15 seconds at a time, stirring after each heating, until chips are melted and mixture is smooth when stirred. Drizzle over top of bars. When drizzle is firm, cut into bars. Store loosely covered at room temperature. Makes 24 to 36 bars.