Line an 8x8-inch square baking pan with parchment paper or foil, leaving an overhang on two sides to create a sling for easy removal. If needed, lightly spray the pan or parchment with nonstick spray to help it stay in place.
1 8x8-inch square baking pan, parchment paper or foil, nonstick spray
In a small microwave-safe bowl, combine the semi-sweet or milk chocolate chips and the vegetable or coconut oil. Microwave in 20–30 second bursts, stirring well after each interval, until the chocolate is completely melted, smooth, and glossy. Set aside to cool slightly; it should be pourable but not hot.
3/4 cup semi-sweet or milk chocolate chips, 1 teaspoon vegetable oil or coconut oil
In a large microwave-safe bowl, combine the white chocolate chips, creamy peanut butter, and unsalted butter. Microwave for 30 seconds, then stir well. Continue microwaving in 20-second bursts, stirring thoroughly after each round, until the mixture is melted and smooth. Avoid overheating; if small lumps remain, let the residual heat melt them as you stir.
2 cups white chocolate chips, 1 cup creamy peanut butter, 2 tablespoons unsalted butter
Stir the vanilla extract and fine sea salt into the warm white chocolate peanut butter mixture until fully combined. Taste carefully and add a pinch more salt if you want extra contrast.
1 teaspoon pure vanilla extract, 1/4 teaspoon fine sea salt
Pour the white chocolate peanut butter mixture into the prepared pan. Use a spatula to spread it evenly and push it into the corners. Gently tap the pan on the counter a few times to help release any air bubbles.
Drizzle or spoon the melted chocolate over the surface of the fudge base in lines or puddles. Using a butter knife or skewer, gently drag through the chocolate and base in horizontal and vertical passes to create a marbled, tiger-stripe pattern. Do not over-swirl or the colors may blend into a muddy shade.
Transfer the pan to the refrigerator and chill for 1½–2 hours, or until the fudge is firm to the touch. If your fridge is very full or warm, it may need a bit longer.
Once fully set, lift the fudge out of the pan using the parchment overhang and place on a cutting board. Use a sharp knife to cut into about 36 small squares (or fewer, larger pieces if preferred). Wipe the knife clean between cuts for the neatest edges.
Serve the fudge slightly chilled or at cool room temperature. Store leftovers in an airtight container at cool room temperature for 3–4 days or in the refrigerator for up to 2 weeks. For longer storage, freeze for up to 2 months.