Line two 12-cup mini muffin pans with paper liners; set aside. In a microwave-safe bowl, combine white chocolate and peanut butter. Microwave, until almost melted, 1 to 1 1/2 minutes, stirring halfway through. Set aside to cool slightly.
Meanwhile, place semisweet chocolate in another microwave-safe bowl. Microwave, stirring once or twice, until almost melted, 2 to 3 minutes.
Dividing evenly, use a spoon to layer semisweet chocolate and peanut-butter mixture into liners, beginning and ending with chocolate; sprinkle with peanuts. Place muffin pans in freezer until peanut butter cups are firm, about 15 minutes. Bring to room temperature before serving.