Line an 8x8 baking dish with parchment paper, be sure to leave a slight overhang to remove the fudge from the pan. (You can use small binder clips to fasten the top edge of the parchment paper to the baking dish. That will help ensure the parchment paper does not fold over on top of the fudge and stick to the top)

Add the white baking chips to a medium size (2-3 quarts) microwave safe bowl. Microwave on full power for 1 minute, stir well and continue to microwave in 30 second intervals until the baking chips are completely melted and smooth.

Add the peanut butter to a small (3-4 cups) microwave safe bowl. Heat at full power for 30 seconds. Stir well and pour into the melted white baking chips. Stir well until combined. (You can heat the white baking chips and peanut butter together, but I prefer to heat them separately)

Pour the melted white baking chips and peanut butter into the lined baking pan. You can tilt the pan, if needed, to evenly distribute the melted white baking chips and peanut butter in the pan.

Next add the milk chocolate chips and vegetable oil to a small (3-4 cups) microwave safe bowl. Microwave on full power for 30 seconds and stir well. If needed, you can microwave for another 15 seconds until the milk chocolate chips are completely melted and smooth. (If the melted milk chocolate is a bit too thick to pour easily, you can stir in 1 teaspoon of vegetable oil)

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

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

Just before you are ready to serve, remove the pan from the refrigerator. Use the parchment paper overhang to remove the fudge from the pan.

Place the fudge on a cutting board. Use a sharp kitchen knife to cut 6 slices x 6 slices. Keep refrigerated until ready to serve.