In a medium mixing bowl stir together peanut butter and softened butter. Add powdered sugar, milk, and vanilla, stirring until well combined. Stir in peanuts. Shape the mixture into two 10-inch rolls; set rolls aside.
In a small saucepan combine chocolate pieces and sweetened condensed milk. Cook over low heat, stirring constantly, until melted and smooth.
Remove saucepan from heat; spread chocolate mixture onto waxed paper. Divide the mixture in half. Working on waxed paper, spread each half into a 10x4-inch rectangle. Place one peanut roll in the center of each chocolate rectangle. Use the waxed paper to help bring the chocolate mixture up and around each of the peanut rolls; press seams to seal.
Wrap candy rolls in waxed paper or clear plastic wrap; chill for 2 to 3 hours or until firm. Unwrap and carefully cut rolls into 1/4-inch slices. Store tightly covered. Makes about 80 slices.