Line an 8×8 baking dish with parchment paper, leaving a little overhang on the sides.

Place the white baking chips into a medium microwave-safe bowl and microwave on full power for 1 minute, then stir. Continue microwaving in 30-second intervals, stirring each time, until smooth.

In a separate small microwave-safe bowl, heat the peanut butter for about 30 seconds until it loosens up, then stir it into the melted white chocolate until uniform.

Pour the white chocolate and peanut butter mixture into the prepared baking dish, spreading it evenly.

Combine the milk chocolate chips with the vegetable oil in a small bowl, then microwave for 30 seconds, stirring until fully melted.

Drizzle the melted milk chocolate over the white chocolate base in a zigzag pattern, then swirl with a knife to create a marbled effect.

Cover the baking dish with aluminum foil and chill in the refrigerator for 45 minutes, or until firm.

Lift the fudge out of the pan using the parchment paper overhang and slice it into a 6×6 grid of squares.