To make marbled chocolate peanut butter bark, line a baking sheet with parchment or waxed paper. Melt 3 cups of white chocolate chips in the microwave, stirring at intervals.
Add 1/2 cup creamy peanut butter and mix until smooth. Spread the mixture on the prepared baking sheet. Melt 1 cup of semi-sweet chocolate chips separately and drop dollops onto the white chocolate mixture.
Swirl with a knife for a marbled effect. Refrigerate until hardened, then break into pieces and store in an airtight container.