Spray a baking sheet with cooking spray, then line it with parchment paper.

Mix white chocolate chips and peanut butter together in a microwave-safe bowl; heat in microwave until half-melted, 30 seconds to 1 minutes. Stir.

Place semi-sweet chocolate chips in a separate microwave-safe bowl; heat in microwave until half-melted, 15 to 30 seconds. Stir in vanilla.

Spread peanut butter mixture onto the prepared baking sheet. Evenly distribute melted semi-sweet chocolate over peanut butter mixture. Using the tip of a sharp knife, drag through both layers to make a marble-pattern.

Refrigerate until set, 30 minutes to 2 hours. Cut into 36 pieces. Store in an airtight container.