Place peanut butter and butter pieces into a medium microwave safe bowl. Microwave on HIGH for 60 seconds. Stir. Microwave an additional 30 seconds. Stir until smooth.
Stir in vanilla extract.
Gradually whisk in powdered sugar.
Pour into a lightly greased 9-inch square pan. Set aside.
In another medium microwave-safe bowl, combine chocolate chips, sweetened condensed milk, and 2 1/2 tablespoons butter.
Microwave on HIGH for 45 seconds. Stir. Microwave another 30 seconds. Whisk until smooth.
Pour on top of peanut butter layer and use a rubber spatula to smooth it out. Refrigerate for at least 2 hours before cutting into squares.
Be sure to buy sweetened condensed milk and not evaporated milk. You will need most of a 14 ounce can. It helps if you sift the powdered sugar to remove any lumps. Otherwise you will get a really good arm workout whisking the lumps out.