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

In a microwave-safe bowl, melt the semi-sweet or dark chocolate chips. Microwave in 30-second intervals, stirring well after each interval, until the chocolate is smooth and fully melted. Alternatively, use a double boiler.

Stir 1/2 teaspoon of the peppermint extract into the melted dark chocolate.

Pour the melted dark chocolate onto the prepared baking sheet. Use a spatula to spread the chocolate evenly into a thin layer (approximately 1/8 to 1/4 inch thick).

Place the baking sheet in the refrigerator for 15 to 20 minutes, or until the chocolate is completely set and firm to the touch.

While the dark chocolate is chilling, melt the white chocolate wafers or chips in a separate microwave-safe bowl using the same 30-second interval technique. If the white chocolate seems too thick, stir in the optional tablespoon of shortening to create a smoother consistency.

Stir in the remaining 1/2 teaspoon of peppermint extract into the melted white chocolate.

Remove the set dark chocolate from the refrigerator. Pour the warm white chocolate directly over the dark chocolate layer. Quickly and gently spread the white chocolate evenly over the base layer.

Immediately sprinkle the crushed candy canes evenly over the surface of the warm white chocolate. Gently press them down to ensure they adhere.

Return the baking sheet to the refrigerator and chill for at least 1 hour, or until both layers are completely hard and firm.

Once fully set, lift the entire slab out of the pan using the parchment paper handles. Peel off the parchment paper and break the bark into irregular pieces by hand, or use a sharp, heavy knife to cut it into squares.