Line a rimmed baking sheet with parchment paper.
Set up a double boiler – if you don’t have a real one, simply set a heat-proof bowl over a small pot of simmering water. Just make sure the bottom of the bowl does not touch the water at all. Add the broken up white chocolate and let it melt slowly, because white chocolate is particularly attitudinal and will seize on you if you try to melt it too quickly. After a minute or two, once a little has started melting, stir continually until the whole thing is smooth.
Once fully melted, spread the white chocolate in an even layer on the prepared pan. Sprinkle evenly with the chopped dark chocolate and crushed peppermint. Let stand until set, which will take several hours. I like to let mine set up in the refrigerator, because I think chilled bark candy tastes much better than room temperature. When it’s ready, break it up into pieces and serve or cover and store for up to 2 weeks.