Grease an 8x8x2-inch square baking pan with butter or shortening and line with parchment paper that overhangs all four sides of the pan (the grease will make the paper stick to the pan).
Combine the semisweet chips, 2 tablespoons of the butter and 1 cup of the milk in a large microwave-safe bowl. Heat at 30 second increments at 100% power until the mixture can be stirred smooth, about 1 minute 30 seconds.
In a separate bowl, combine the peanut butter chips, the remaining 2 tablespoons butter, and the remaining sweetened condensed milk. Use a small rubber spatula to efficiently remove all of the milk from the can. Heat in the microwave in 30 second increments at 100% power until the mixture can be stirred smooth, about 1 minute.
Pour the chocolate mixture into the prepared pan followed by the peanut butter mixture on top. Use a butter knife to swirl the peanut butter portion into the chocolate portion.
Refrigerate until firm, about 2 hours. Lift the candy from the pan using the overhanging parchment paper. Cut into 2-inch squares.
Package in a festive air-tight tin or in cellophane bags tied with ribbon for gift-giving.