Line a 9-inch x 13-inch pan or baking dish with foil or parchment paper, being sure to come up the sides of the pan. Set aside.
Heat the white chocolate chips in a microwave-safe bowl for 1 minute and then stir. Add the peanut butter and heat for an additional 30 seconds. Stir until fully melted, creamy, and fully combined. Repeat heating in 30-second intervals as needed.
Pour the peanut butter fudge into the prepared pan and set aside.
In another microwave-safe bowl, heat the chocolate chips for 1 minute and then stir. Continue heating in 30-second intervals and then stirring until melted. (If the chocolate is too thick to run off of a spoon, stir in 1 tablespoon of shortening to thin the chocolate.)
Drizzle chocolate in parallel lines over the peanut butter fudge. Then, take a knife and drag it through the chocolate, going in the opposite direction of the lines and swirling the chocolate until you are happy with the pattern in the fudge. You want it to sort of look like tiger stripes.
Let the fudge sit on the counter for 2 hours to set up. (You can also chill the fudge in the fridge for a firmer texture.
Use the foil/parchment paper to lift the fudge out of the pan. Peel the foil/paper away and use a sharp knife to cut it into 64 pieces.
Notes
Recipe yield and nutritional information will vary based on how big you cut the pieces of fudge.
Store any leftovers in an airtight container in the fridge.