Have ready a rimmed, 9x13" pan lined with parchment paper.
Add the 9 ounces of chopped, semi-sweet melting chocolate to a large microwave-safe bowl. Melt the chocolate at 30-second intervals, stirring after each stint in the microwave, until it has reached a temperature of 122°.
Begin adding semi-sweet seed chocolate a handful at a time, stirring until melted after each addition. Take the temperature after each addition to gauge when you are close to working temperature, 86°-90°.
Once the semi-sweet chocolate has reached working temperature, stir in a ¼ teaspoon of the peppermint oil, and 2 tablespoons of crushed peppermint. Transfer the chocolate to the prepared pan and spread into a thin layer. Set aside until ready to use.
Add the 9 ounces of chopped white melting chocolate to a large microwave-safe bowl. Melt the chocolate at 30-second intervals, stirring after each stint in the microwave, until it has reached a temperature of 105°.
Begin adding white seed chocolate a handful at a time, stirring until melted after each addition. Take the temperature after each addition to gauge when you are close to working temperature, 84°-86°.
Once the white chocolate has reached working temperature, stir in a ¼ teaspoon of the peppermint oil and pour over the semi-sweet chocolate. (see note below if semi-sweet chocolate has already set).
Use a small, offset spatula spread the white chocolate evenly across the semi-sweet chocolate. Sprinkle with the remaining crushed peppermint and transfer to the refrigerator to set about 30 minutes.
Break apart, serve and enjoy.
Notes
If the semi-sweet chocolate layer happens to set before you have finished seeding your white chocolate, use a blow dryer to heat the dark chocolate layer before you add your white chocolate layer. The heat from the dryer will melt the dark chocolate just enough so the layers will bind.