
Line a 9"×12" sheet pan with parchment paper.
In a microwave-safe bowl, melt the dark chocolate until smooth and glossy, in 30-second intervals for 2–3 minutes, stirring after each interval. Pour the melted dark chocolate on the parchment paper and use a rubber spatula to spread it out evenly. Refrigerate until it sets, about 30 minutes.
In another microwave-safe bowl, melt the white chocolate until smooth and glossy, in 30-second intervals for 2–3 minutes, stirring after each interval. Stir in half of the crushed peppermint candy and the peppermint extract, if using. Pour the white chocolate on top of the solid dark chocolate and spread it out evenly. Sprinkle the remaining crushed peppermint candy on top.
Place back in the refrigerator until set, about 1–2 hours. Remove from the parchment paper, break into smaller pieces, and enjoy.