Line a rimmed baking sheet (about 9×13 inches) with parchment paper, letting it overhang the sides so you can lift the bark out later. Decide how thick you want your bark: for thinner bark spread closer to the full 9×13; for thicker bark keep it closer to a 9×9 area on the sheet.

Place the candy canes or peppermint candies in a zip-top bag and seal. Set the bag on a folded kitchen towel and gently crush with a rolling pin or the bottom of a heavy saucepan until you have a mix of small pieces and some fine peppermint “dust.” Set aside.

Add the dark chocolate and 1 teaspoon oil (if using) to a microwave-safe bowl. Microwave in 20–30 second bursts, stirring well after each, until just melted and smooth, about 1 1/2–2 minutes total. Alternatively, melt in a heatproof bowl set over a pot of barely simmering water, stirring until smooth.

Pour the melted dark chocolate onto the prepared baking sheet. Use an offset spatula or the back of a spoon to spread it into an even rectangle about 1/4 inch thick. Gently tap the pan on the counter a few times to release air bubbles. Refrigerate for 10–15 minutes, until just set but not rock-hard; the surface should be mostly firm but still very slightly tacky.

While the dark chocolate chills, place the white chocolate and 1 teaspoon oil (if using) in a clean microwave-safe bowl. Microwave in 20–30 second bursts, stirring well between each, until mostly melted with a few small lumps remaining. Remove from the microwave and stir until completely smooth.

Stir 1/2 teaspoon peppermint extract into the melted white chocolate. Taste a tiny amount (careful, it’s warm) and add up to another 1/2 teaspoon, to taste, being careful not to overdo it.

Remove the pan with the dark chocolate from the fridge. Pour the melted white chocolate over the dark chocolate layer and quickly spread it into an even layer with a spatula, working gently so you don’t disturb the dark chocolate underneath. For a marbled effect instead of clean layers, drizzle the white chocolate in lines and swirl gently with a toothpick.

Immediately sprinkle the crushed candy canes evenly over the surface of the white chocolate before it sets. If using, add a light pinch of flaky sea salt over the top. Gently press the candy pieces into the chocolate with clean hands so they adhere.

Refrigerate the pan for 30–40 minutes, or until the bark is completely set and firm in the center. Lift the bark out of the pan using the parchment paper and place it on a cutting board. Break into roughly 2–3 inch pieces by hand, or use a large, sharp knife to cut if you prefer cleaner lines. Store in an airtight container.