Line an 8 x 8-inch baking pan with aluminum foil and lightly grease it with butter or cooking spray. The foil should cover the pan neatly so you can lift the chilled fudge out without sticking.

Place 7 ounces of sweetened condensed milk, the semi-sweet chocolate chips, and 1 tablespoon of butter in one microwave-safe bowl. Keep this bowl separate so the chocolate layer melts evenly and stays distinct from the peanut butter layer.

Place the remaining 7 ounces of sweetened condensed milk, the peanut butter chips, and the remaining 1 tablespoon of butter in a second microwave-safe bowl. Having two bowls ready before heating makes the swirling step much easier later.

Microwave both bowls on high in 30-second bursts, stirring well after each round, until the chips are melted. The mixtures should change from stiff and matte to glossy and fluid, and the aroma should stay sweet rather than toasted.

Stir 1 teaspoon of vanilla extract and 4 ounces of cream cheese into each bowl, then microwave both bowls for 30 seconds on high again. After stirring, each mixture should look smooth, creamy, and thick with no visible cream cheese bits.

Spread about two-thirds of the chocolate mixture into the prepared pan, then spoon half of the peanut butter mixture over it. Use scattered spoonfuls instead of one heavy layer so the colors stay defined when you swirl them.

Add the remaining chocolate mixture over the peanut butter layer, then finish with the remaining peanut butter mixture on top. The layered dollops should look uneven in a good way, which gives the swirl pattern more contrast.

Drag a knife through the fudge to make swirls, then refrigerate for 3 hours before serving. Once chilled, the surface should feel firm with a little soft resistance when pressed before cutting.