Line a baking sheet with parchment paper, making sure it covers the edges completely.
Melt the semi-sweet chocolate in a double boiler or microwave. If using a microwave, heat in 30-second intervals, stirring after each, until smooth and fully melted.
Pour the melted semi-sweet chocolate onto the prepared baking sheet. Use a spatula to spread it evenly into a rectangle, about ¼ inch thick. Refrigerate for about 10 minutes, or until set.
While the semi-sweet layer is chilling, melt the white chocolate in the same manner as the semi-sweet chocolate.
Once the white chocolate is melted, remove the baking sheet from the refrigerator. Pour the melted white chocolate over the set semi-sweet chocolate layer.
Using a spatula, spread the white chocolate carefully to cover the entire surface.
Sprinkle the crushed peppermint candies evenly over the white chocolate layer. If desired, drizzle a few drops of peppermint extract over the top for extra flavor.
Refrigerate the entire sheet for 20-30 minutes, or until the chocolate is completely set.
Once set, remove from the refrigerator and break into pieces or cut into squares.