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What you need:
360 g white chocolate chopped coarsely
½ cup (140g) smooth peanut butter
400 g dark chocolate chopped coarsely
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Tip for coarsely chopping chocolate - bash chocolate with a meat mallet while still in its wrapping
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Melt white chocolate in a bowl over a pot of simmering water.
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Cool for 5 minutes then add peanut butter, stir until smooth.
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Melt the dark chocolate the same way. Cool slightly
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Drop alternate spoonfuls of the white chocolate mixture and dark chocolate mixture into a small pan you greased and lined with parchment paper earlier. Gently shake pan to level mixture. Can you tell I didn't wait long enough for the dark chocolate to cool? 
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Then drag a skewer back and forth through the mixture several times for a marbled affect.
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Now for the hard part: Let stand at room temperature for 2 hours or until set.
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If you're impatient like me, you can stick it in the fridge.
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When it has set, cut into small squares.
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Yummy 
It's delicious and looks like you spent more than 20 minutes on it.