Line a 9x13 inch baking sheet with parchment paper, ensuring the paper hangs over the sides slightly. Crush the candy canes thoroughly by placing them in a zip-top bag and using a rolling pin. Set the crushed candy aside.

Melt the semi-sweet or dark chocolate chips. Use a microwave (stirring every 30 seconds) or a double boiler until completely smooth.

Stir 1/2 teaspoon of the peppermint extract into the melted dark chocolate. Pour the chocolate onto the prepared baking sheet and spread evenly into a thin layer (about 1/8 inch thick).

Place the baking sheet in the refrigerator for 15 to 20 minutes, allowing the dark chocolate to fully set and become firm to the touch.

While the dark chocolate chills, melt the white chocolate wafers or chips in a separate bowl using the same technique (30-second intervals). If the chocolate seems too thick, stir in the optional 1 tablespoon of shortening for a smoother consistency.

Stir the remaining 1/2 teaspoon of peppermint extract into the melted white chocolate. Remove the dark chocolate from the refrigerator and pour the warm white chocolate directly over the set layer.

Quickly spread the white chocolate evenly over the dark chocolate. Immediately sprinkle the crushed candy canes evenly over the surface, gently pressing them into the warm chocolate to ensure adhesion.

Return the baking sheet to the refrigerator and chill for at least 1 hour, or until both layers are completely hard. Once set, lift the slab using the parchment paper handles, peel off the paper, and break the bark into irregular pieces or cut into squares using a sharp knife.