Line a 20 x 30cm baking tin with baking paper. Find two bowls, and in the first melt the white chocolate and peanut butter together in a microwave stirring every 30 seconds until nice and creamy. Pour the mixture in the baking tin and spread evenly across its base.
In the second bowl, repeat the process with any leftover chocolate, stirring every 30 seconds until smooth. Drizzle this chocolate over the peanut butter mixture.
With a knife, cut through in swirls. Refrigerate until set, then cut into shards to serve.