Line an 8x8-inch pan with parchment paper. Combine peanut butter and sweetened condensed milk in a microwave-safe bowl. Heat in 30-second intervals, stirring until smooth. Stir in vanilla extract. Pour mixture into prepared pan and spread evenly. Refrigerate for at least 2 hours until firm. Slice and serve.
Notes
Use creamy peanut butter for best texture. Chill fully before cutting.