Line a 13 x 18" sheet pan with parchment paper.
Fill the bottom of a large pot with about 3 inches water and bring to a simmer over medium heat. Reduce heat to low, then place a heat-proof bowl or another pot on top.
Add the semi-sweet chocolate and stir occasionally until chocolate melts.
Stir in 1/2 tsp. peppermint extract. Pour the chocolate into the prepared sheet pan and spread using an offset spatula creating an even layer about 1/8" thick. Refrigerate until set; about 20 minutes.
Over the same pot of barely simmering water, place another pot or heat-proof bowl. Add the white chocolate and stir until melted. Pour over the 'set' semi-sweet chocolate spreading evenly, then top with crushed peppermint candy. Press the peppermint lightly into the warm white chocolate.
Refrigerate until set; about 20 minutes. When set, break into uneven pieces.