Line a sheet pan with parchment paper. Melt the white chocolate chips in a microwave-safe bowl, carefully checking and stirring the chocolate every 30 seconds or so to avoid scorching. Once melted and smooth, immediately stir in the mint extract, 1/2 cup of the crushed candies and the crushed cookies, if using. Immediately pour the mixture onto the prepared pan and spread evenly (I use an offset spatula). Refrigerate until firm.
Once the white chocolate is firm, make the top layer. Melt the bittersweet chocolate in a microwave-safe bowl, checking and stirring every 30 seconds or so to avoid scorching. Once melted and smooth, spread the bittersweet chocolate evenly over the white chocolate. Sprinkle the remaining 1/4 cup of crushed peppermint candies on top.
Refrigerate until firm. Break into pieces and store in the refrigerator.