Place the bittersweet chocolate in a heatproof bowl, and set the bowl over a pan of gently simmering water (the bottom of the bowl should not be touching the water). Melt the chocolate, stirring occasionally, until smooth.
Meanwhile, line a baking sheet with foil and place the cookie cutters on the foil. Pour the melted bittersweet chocolate into the cookie cutters (I poured the chocolate into a piping tool to make this step easier). Use the pointed end of a chopstick to get the chocolate into all the nooks and crannies of the cookie cutters. The chocolate should be about 1/8 inch thick. If you don't have many cookie cutters to fill at once, then you will need to make these in batches - set aside any unused chocolate for the next set.
the baking sheet/s in the freezer and chill for 20 minutes. Meanwhile,
melt the white chocolate; once melted and smooth, remove from the heat
and let cool for 5 minutes before whisking in the peppermint extract.
(Adding the extract while the chocolate is too hot may cause the
chocolate to seize up.)
Remove the baking sheet from the freezer and let stand at room temperature for 5 minutes. (If you try to pour the white chocolate over the dark chocolate straight from the freezer, then it will harden too quickly before you can spread it out, so allowing the dark chocolate to warm up slightly will make this next step easier.)
white chocolate has a thicker consistency, but try to use the same
amount as you did of dark chocolate, so that it comes out evenly. Pour
the white chocolate over the dark chocolate, using a chopstick again to
spread out the chocolate to the edges and corners of the cookie
cutters. The white chocolate should also be 1/8 inch thick, so that now
you have a 1/4 inch thick piece of bark with both layers of chocolate.
Before the white chocolate sets, immediately sprinkle with the crushed
peppermint, and add a tiny sprinkle of sea salt, before moving on to the
next one. Cover any unused white chocolate until you're ready to make
the next batch.
Place the baking sheet in the freezer again to thoroughly chill, about 30 minutes. Remove from the freezer. Lift the cookie cutters off the foil and carefully loosen the chocolate from the cutters, using the blunt end of a chopstick to help push it out of the mold. Handling the chocolate as little as possible, so as not to melt it with your fingers, transfer the pieces of bark to a clean baking sheet and place in the refrigerator.
Follow the whole process again until you've used up all the chocolate. Alternatively, instead of pouring the chocolate into the cookie cutters, you can simply spread the chocolate evenly on a baking sheet, and then break into pieces after it's chilled.
For a gift, place the bark into a tissue-lined box and tie with a ribbon. A candy cane on top looks pretty.