1-First Step: Pan Prep and Mise en Place Line an 8×8-inch baking pan with aluminum foil, leaving an overhang on all sides so you can lift the fudge out easily later. Lightly grease the foil with butter or non-stick cooking spray to prevent sticking. Arrange measuring cups, a heatproof spatula, two medium microwave-safe bowls, a sharp knife for swirls, and ingredients on the counter so everything is within reach.

2-Second Step: Prepare the Chocolate Mixture Measure half the sweetened condensed milk (7 ounces) into a microwave-safe bowl. Add 2 1/2 cups semi-sweet chocolate chips and 1 tablespoon butter. Microwave on high in 30-second intervals, stirring well after each interval to avoid overheating. Most microwaves will need about 1 to 2 minutes total; stop when the chips are melted and the mixture is smooth. Stir in 1 teaspoon vanilla extract and 4 ounces of softened cream cheese, mixing until fully combined and silky. If small lumps of cream cheese remain, microwave an additional 10 to 20 seconds and stir again.

3-Third Step: Prepare the Peanut Butter Mixture In a separate microwave-safe bowl, combine the remaining 7 ounces sweetened condensed milk, 10 ounces peanut butter chips, and 1 tablespoon butter. Microwave in 30-second bursts, stirring between each burst until the chips melt and the mixture is uniform. Add the remaining 1 teaspoon vanilla extract and the other 4 ounces of cream cheese, stirring until smooth. A short 30-second burst after adding cream cheese helps everything blend into a glossy peanut butter cheesecake layer.

4-Fourth Step: Layer the Fudge Pour two-thirds of the chocolate mixture into the prepared pan and spread it out with a spatula to an even layer. Spoon half of the peanut butter mixture on top in dollops. Follow with the remaining chocolate and peanut butter mixtures, alternating to create visible bands of chocolate and peanut butter. This alternating layering technique helps you get a pretty marble once you swirl.

5-Fifth Step: Swirl for Marble Effect Use a butter knife, skewer, or the tip of a spoon to drag through the layers in gentle zigzags or circular motions, making swirls without overmixing. The goal is pretty marbling, not a uniform blend; leave distinct chocolate and peanut butter streaks for the best look and flavor contrast.

6-Final Step: Chill, Slice, and Serve Refrigerate the pan for at least 3 hours, or overnight for firmer slices. When set, lift the fudge out using the foil overhang and peel the foil away. Use a sharp knife warmed with hot water and wiped dry between cuts for clean squares. Store leftovers in an airtight container in the refrigerator for up to one week, or freeze for longer storage.