01 - Line an 8-inch square baking pan with parchment paper, leaving overhang on sides for easy removal of fudge later.
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 constantly until completely smooth and glossy.
03 - Pour the melted chocolate mixture into the prepared pan, spreading evenly with a spatula. Refrigerate for 30 minutes until firm but not completely hard.
04 - In a medium saucepan over low heat, melt peanut butter and butter together, stirring until blended. Remove from heat and whisk in powdered sugar and vanilla extract until smooth and creamy.
05 - Pour the peanut butter mixture over the chilled chocolate layer. Spread evenly with a spatula, ensuring full coverage.
06 - Refrigerate for at least 2 hours or until completely firm throughout.
07 - Using the parchment paper overhang, lift the fudge block from the pan. Cut into 16 equal squares with a sharp knife.