2Melt white chocolate in the top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching.
3Place melted white chocolate in a large bowl and stir in peanut butter until well combined. Pour mixture into the prepared baking dish and smooth the top with a spatula.
4Melt dark chocolate in the top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching. Drizzle melted dark chocolate over the white chocolate-peanut butter mixture in stripes. Use a butter knife to make a marble pattern.
5Allow candy to set until it begins to firm up but is not solidified, about 45 minutes. Cut into squares.