Line an 8x8-inch baking pan with parchment paper, leaving overhang on two opposite sides to create handles. Lightly mist with nonstick spray if desired for easier release.
In a large microwave-safe bowl, combine the white chocolate chips, 1 cup peanut butter, and 1 tablespoon unsalted butter. Microwave on 50% power for 60 seconds, then stir very well. Continue microwaving at 50% power in 20–30 second intervals, stirring after each burst, until the mixture is smooth and fully melted.
2 cups white chocolate chips, 1 cup creamy peanut butter, 1 tablespoon unsalted butter
Once the mixture is silky and lump-free, stir in the vanilla extract and fine sea salt. Taste a tiny bit carefully and adjust the salt if needed. The mixture should be thick but pourable.
1 teaspoon pure vanilla extract, 1/4 teaspoon fine sea salt
Scrape the white chocolate peanut butter mixture into the prepared pan. Use a spatula to spread it into an even layer all the way to the corners. Tap the pan gently on the counter to release any air bubbles.
In a small microwave-safe bowl, combine the semi-sweet or dark chocolate chips and 2 teaspoons peanut butter. Microwave at 50% power for 30 seconds, then stir. Continue microwaving in 15–20 second bursts, stirring between each, until melted and glossy. If the mixture is too thick to drizzle, stir in 1 teaspoon neutral oil, adding up to 1 teaspoon more only if needed to reach a smooth, drizzleable consistency.
1 cup semi-sweet or dark chocolate chips, 2 teaspoons creamy peanut butter, 1-2 teaspoons neutral oil
Quickly spoon or drizzle the melted chocolate mixture over the white fudge layer in thick lines or random patches. Using a butter knife or skewer, gently drag through both layers in curving motions to create tiger-like stripes. Make just a few passes so the colors stay distinct and don’t blend into one muddy shade.
If you like a sweet-and-salty finish, sprinkle a light pinch of flaky sea salt evenly over the top of the swirled fudge.
Flaky sea salt
Cover the pan lightly with plastic wrap or foil and refrigerate for 1½–2 hours, or until the fudge is fully set and firm to the touch.
Use the parchment handles to lift the fudge slab out of the pan and place it on a cutting board. With a sharp knife, cut into 25–36 small squares, wiping the blade clean between cuts for neat edges.
Let the fudge sit at room temperature for 5–10 minutes before serving so the texture is creamy and fudgy rather than hard. Arrange on a serving plate, platter, or in candy cups.