Line a large baking sheet (approximately 10x15 inches or a similar size) with parchment paper or a silicone baking mat. Ensure the lining extends over the sides to make removing the finished bark easier. Set the prepared pan aside.

Place the peppermint candy canes into a heavy-duty zip-top bag. Seal the bag, pressing out as much air as possible. Using a rolling pin, a meat tenderizer, or the bottom of a heavy pan, carefully smash the candies until they are crushed into a mixture of small and medium-sized pieces. Set the crushed candy aside in a small bowl.

Combine the dark chocolate and the optional coconut oil or shortening in a large, heat-proof bowl. Melt the chocolate using the microwave (30-second intervals, stirring well) or the double boiler method until completely smooth.

Pour the melted dark chocolate onto the center of the prepared baking sheet. Use a spatula or offset knife to spread the chocolate evenly into a thin layer (approximately 1/8 to 1/4 inch thick). Transfer the baking sheet to the refrigerator and chill for 20 to 30 minutes, or until the layer is completely set and firm to the touch.

Place the white chocolate into a clean, dry, heat-proof bowl. Melt the white chocolate using the microwave (15-second intervals) or double boiler over very low heat. Once the white chocolate is nearly smooth, remove it from the heat source and stir in the 1/2 teaspoon of peppermint extract until the mixture is fully incorporated and smooth.

Pour the melted white chocolate over the solidified dark chocolate layer. Quickly use a spatula or offset knife to spread the white chocolate evenly over the dark chocolate. Immediately and generously sprinkle the reserved crushed candy canes over the top of the wet white chocolate layer. Gently press the peppermint pieces into the white chocolate with the back of a spoon or your fingertips to ensure they adhere firmly.

Return the baking sheet to the refrigerator and chill for at least 1 to 2 hours, or until both layers of the peppermint bark are completely firm and solid.

Remove the bark from the refrigerator. Lift the entire slab of bark off the baking sheet using the parchment paper lining. Place the bark on a cutting board, peel the parchment paper away, and use your hands to break the chocolate slab into irregular, bite-sized pieces or use a large, sharp knife to cut it into neat squares.