Prep a half sheet pan by adding either parchment paper or a silicone mat.
In one medium bowl, melt bittersweet chocolate over a pan with 1 inch simmering water. Careful not to get water in the chocolate or get it too hot.
Remove from pan and pour onto prepared pan. Spread with an offset spatula leaving a ½ edge uncovered around all sides. Set in refrigerator for 8-10 minutes. Don’t allow it to fully set. We want the edges set and the middle still a little glossy. If it’s completely set, the layers won’t stay together.
While that’s cooling, in another medium bowl add white chocolate and peppermint extract. Place over pan of simmering water and melt gently. (White chocolate has a tendency to melt faster and overheat, so make sure you take it off the heat occasionally to not burn it.)
Once melted, remove the pan from the refrigerator and quickly pour and spread the white chocolate over the bittersweet chocolate. Do this by placing all the white chocolate at one short edge of the bittersweet, then using an offset, quickly spreading it in one direction. We don’t want to melt the chocolate underneath and make a swirl pattern; we want two distinctive layers.
Sprinkle crushed peppermints over top and allow to chill in the refrigerator for another 30 minutes.