Line a 9x9-inch baking dish with parchment paper or aluminum foil, leaving an overhang for easy removal.
Place the semi-sweet chocolate chips in a microwave-safe bowl. Microwave in 30-second intervals, stirring after each, until smooth. Set aside.
Place the vanilla chips in a separate microwave-safe bowl. Microwave in 30-second intervals, stirring after each, until melted and smooth.
Stir the peanut butter into the melted vanilla chips until fully combined and creamy.
Spread the vanilla-peanut butter mixture evenly into the prepared pan.
Drizzle the melted chocolate over the top in lines. Use a knife or skewer to gently swirl the chocolate into the vanilla mixture to create a marbled pattern.
Let the candy cool completely at room temperature or refrigerate until firm.
Lift the candy out of the pan using the parchment or foil, then cut into 24 squares.
Store in an airtight container at room temperature for up to 1 week or refrigerate for longer storage.