01 - Line an 8-inch square baking pan with parchment paper, allowing overhang on opposite sides for easy removal.
02 - Combine chocolate chips, sweetened condensed milk, and butter in a heatproof bowl. Melt using a double boiler or microwave in 30-second intervals, stirring frequently until completely smooth.
03 - Pour melted chocolate mixture into prepared pan, spreading evenly. Refrigerate for 30 minutes or until firm to the touch.
04 - Melt peanut butter and butter in a medium saucepan over low heat. Remove from heat and whisk in powdered sugar and vanilla extract until smooth and creamy.
05 - Pour peanut butter mixture over the chilled chocolate layer. Use an offset spatula to spread evenly and create a smooth surface.
06 - Refrigerate for at least 2 hours or until completely firm throughout.
07 - Lift fudge from pan using parchment overhang. Cut into 16 equal squares with a sharp knife, wiping blade clean between cuts.