Line a 13"x 18" baking sheet (or similar size) with parchment paper and set aside.
Melt dark chocolate using a double-boiler method: Bring a small saucepan of water to a boil with heatproof bowl over the top. Add the chocolate to the bowl and slowly melt and stir the chocolate until almost all of it has melted. Remove from heat, let sit a few minutes and then whisk it all together. Alternatively you could melt the chocolate in the microwave at 20 second intervals, whisking in between until it's fully melted.
Pour out chocolate and lightly spread into a rectangle onto parchment lined baking sheet. Set in the freezer for 5-10 minutes until set.
Meanwhile, melt white chocolate and extract together. Once dark chocolate is chilled, pour the white chocolate on top and carefully spread it over.
Sprinkle crushed candy canes and sea salt (if using) and let sit at room temperature for a few hours or place in the freezer for about 5 minutes until bark hardens.
Break into pieces and wrap up to give away as gifts, or store for serving later.
Bark will keep stored, at cool room temperature for about 2-3 weeks, depending on the ingredients added to the bark. A good rule of thumb to note is that if you add ingredients that keep well at room temperature for a while, the bark will keep well.
This is roughly 8-12 servings but will make about 4 gift bags of bark, depending on how much you are placing in a package.