Cut a piece of foil big enough to fit inside and over the edges of an 8 x 8 x 2" baking dish or pan. Place foil in the pan and press against bottom, sides and corners to fit pan. Spray with no stick cooking spray.
In a large microwave bowl, stir together the powdered sugar, milk and salt.
Add the butter and microwave uncovered for 1 minute on high.
Remove from microwave and stir. Microwave again on high for 1-1/2 minutes.
Remove from microwave and stir. Microwave on high for 1-1/2 minutes longer or until bubbly.
Remove from microwave and quickly add chocolate chips, peanut butter and vanilla. Stir until blended.
Pour into foiled pan and smooth into an even layer. Refrigerate at least 2 hours.
Cut into squares and store in an air tight container in the refrigerator.