Preheat oven to 350°F.
Combine flour, butter and 1/2 cup powdered sugar in a medium bowl, cutting in butter with pastry blender or two forks. Mix until crumbly. Press into the bottom of an ungreased 13 x 9-inch pan. Bake for 15 to 18 minutes or until lightly browned. Cool 3 minutes.
Melt 1 cup of the peanut butter chips and corn syrup in a large micro-safe bowl. Heat on HIGH power for 1 minute, stirring occasionally until chips are melted. Beat in the eggs, peanut butter, 1/2 cup powdered sugar and 1/2 cup peanut butter chips. Pour into crust. Top with remaining peanut butter chips. Bake for 18 to 22 minutes or until filling is set. Cool 5 to 10 minutes on wire rack.
Melt white chocolate chips and shortening in a microwave-safe bowl on MEDIUM power for about 1 minute, stirring occasionally until melted. Spread over bars. Cool completely before cutting.
Decorating tip: To make a colored topping, reserve 3 tablespoons of the melted white chocolate and stir in a small amount of paste food coloring. Frost the bars with the white chocolate and then drizzle with the colored white chocolate. Use a knife to swirl the colors. Or, get really creative and use several different colors!