Crush the peppermints into a powder by placing them in a ziplog bag and hammering them with something heavy like a rolling pin. or say, a hammer.
Carefully melt the semisweet chocolate and white chocolate in separate bowls over hot water or slowly in the microwave, using the lowest setting for about 1 1/5 minutes, then stir to fully melt.
Pour half of the semisweet chocolate into the prepared pan and freeze for 5 minutes.
Add the crushed peppermints and the drop of green food coloring to the melted white chocolate and stir until the color is evenly distributed; add more color if desired. Pour the mixture over the cold semisweet chocolate layer. Freeze for 5 minutes or until set.
Pour the remaining half of the semisweet chocolate on top of the green filling layer and spread it evenly.
Place the finished pan in the freezer for another 5 minutes or until set.
To cut, remove the foil from pan and slice the layered chocolate into desired shaped. The mints will keep for several days in an airtight container in the fridge. Separate each layer with waxed paper.