01 - Line a 9 x 13 inch baking sheet with parchment paper.
02 - Melt the dark or semisweet chocolate in a heatproof bowl over simmering water or in the microwave in 20-second intervals, stirring until smooth.
03 - Mix in half the peppermint extract if using. Pour the melted chocolate onto the prepared baking sheet and spread evenly to about 1/4 inch thickness. Chill until firm, approximately 20 minutes.
04 - Using the same melting technique, melt the white chocolate.
05 - Stir in remaining peppermint extract if desired. Pour the melted white chocolate over the set dark chocolate layer, spreading gently to cover uniformly.
06 - Immediately sprinkle crushed peppermint candies evenly over the white chocolate, pressing lightly to adhere.
07 - Refrigerate until completely firm, about 30 minutes.
08 - Break into pieces or cut into squares and serve.