Line a 9x13 inch baking sheet with parchment paper, ensuring the paper hangs over the edges. Crush the candy canes into small pieces using a zip-top bag and a rolling pin. Set aside.

In a microwave-safe bowl, melt the semi-sweet or dark chocolate chips. Microwave in 30-second intervals, stirring after each, until smooth. Stir in 1/2 teaspoon of the peppermint extract.

Pour the melted dark chocolate onto the prepared baking sheet and spread evenly into a thin layer (1/8 to 1/4 inch thick). Place the sheet in the refrigerator for 15 to 20 minutes, or until the layer is completely set and firm.

While the dark chocolate chills, melt the white chocolate wafers in a separate microwave-safe bowl using 30-second intervals. If the white chocolate is too thick, stir in the optional tablespoon of shortening. Stir in the remaining 1/2 teaspoon of peppermint extract.

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

Immediately sprinkle the crushed candy canes evenly over the warm white chocolate layer. 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.

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