Line a 9x13-inch / 23x33cm sheet pan with parchment paper, leaving a slight overhang on two sides for easy lifting later.

Add the sugar-free dark chocolate chips to a microwave-safe bowl. Microwave in 30-second bursts, stirring after each, until fully melted and smooth, about 90 seconds total (chocolate should reach 45 to 50 C / 113 to 122 F).

Stir 1/4 tsp of the peppermint extract into the melted dark chocolate, then pour it onto the lined sheet pan. Spread into an even layer, about 6 mm / 1/4 inch thick, with an offset knife or the back of a spoon.

Melt the sugar-free white chocolate in a separate microwave-safe bowl the same way, in 30-second bursts, stirring between each, until smooth.

Stir the remaining 1/4 tsp peppermint extract into the white chocolate, then drop spoonfuls over the dark chocolate layer.

Drag a knife or skewer through both layers in loose swirls to create a marbled pattern. Don't overmix or the colors will blend into gray.

Sprinkle the crushed sugar-free candy canes evenly over the top, then add a light pinch of flaky sea salt if using.

Transfer the sheet pan to the fridge and chill for 30 to 45 minutes, until firm and cold with no give when pressed.

Lift the bark out using the parchment overhang and break into uneven pieces by hand or with the back of a knife.

Serve right away or store the pieces in an airtight container.