Line an 8x8 baking dish with parchment paper, leaving a slight overhang to remove the fudge. You can use small binder clips to fasten the top edge of the parchment paper to the baking dish to prevent it from folding over.

Add the white baking chips to a medium microwave-safe bowl. Microwave on full power for 1 minute, stir well, and continue to microwave in 30-second intervals until completely melted and smooth.

Add the peanut butter to a small microwave-safe bowl. Heat at full power for 30 seconds, stir well, and pour into the melted white baking chips. Stir until combined.

Pour the melted white baking chips and peanut butter into the lined baking pan. Tilt the pan to evenly distribute.

Add the milk chocolate chips and vegetable oil to a small microwave-safe bowl. Microwave on full power for 30 seconds and stir well. If needed, microwave for another 15 seconds until completely melted and smooth. If the melted milk chocolate is too thick, stir in 1 teaspoon of vegetable oil.

Pour the melted milk chocolate over the white chocolate and peanut butter in zigzag lines.

Using a knife, create a swirl design by dragging the tip of the knife around the pan. Cover tightly with aluminum foil and chill in the refrigerator for 45 minutes.

Just before serving, remove the pan from the refrigerator. Use the parchment paper overhang to lift the fudge out of the pan.

Place the fudge on a cutting board and use a sharp knife to cut into 6 slices by 6 slices. Keep refrigerated until ready to serve.