Line a 9x13-inch baking pan or a rimmed baking sheet with parchment, letting it overhang on two sides for easy lifting. Clear a flat shelf in the fridge for quick setting.

Place the chopped dark chocolate in a microwave-safe bowl. Microwave in 20 to 30 second bursts, stirring well after each, until mostly melted with a few small pieces remaining, about 1 1/2 to 2 minutes total. Stir until completely smooth and slightly warm to the touch, not hot.

Stir in a pinch of fine sea salt to the dark chocolate if using. Pour onto the parchment and spread into an even rectangle about 1/4-inch thick, roughly 8x12 inches. Tap the pan on the counter to level.

Refrigerate the dark layer for 8 to 10 minutes until it looks set and matte but still yields slightly when pressed with a fingertip. Do not let it get rock-hard, or the layers may separate.

While the dark layer sets, place the chopped white chocolate in a clean, dry microwave-safe bowl. Microwave in 15 to 20 second bursts, stirring well, until mostly melted, 1 to 2 minutes total. Stir until silky. If using peppermint extract, add a few drops to the white chocolate and stir thoroughly.

Let the melted white chocolate stand 2 to 3 minutes to cool slightly; it should be warm and pourable, not steaming. Pour it over the softly set dark layer and spread gently to the edges with an offset spatula.

Immediately sprinkle the crushed candy canes evenly over the white chocolate. Lightly press the larger pieces so they adhere without sinking.

Refrigerate the pan for 20 to 30 minutes, just until firm to the touch. Avoid chilling for hours, which can cause condensation when brought back to room temperature.

Lift the bark out of the pan using the parchment overhang. Place on a cutting board and let stand 5 minutes at room temperature.

Break the bark into shards with your hands or cut into squares with a sharp chef’s knife. If cutting, warm the knife under hot water and dry it between cuts for the cleanest edges.