Prepare a large baking tray with a silicone baking mat or a sheet of baking parchment, and set aside until ready to use.

Melt milk chocolate over a double boiler or bain-marie. Stir a few drops of peppermint oil (if using) into the melted chocolate.

Meanwhile, melt white chocolate or green candy melts in a separate bowl. If using white chocolate, stir a few drops of oil-based green food coloring into the melted chocolate to achieve your desired shade of green.

Pour melted milk chocolate onto prepared baking sheet, and use a stepped palette knife to spread to about 1/4-inch / 6 millimeters thick.

Immediately drizzle green chocolate on top of the milk chocolate, and use a knife or skewer to create a marble pattern.

Working quickly, decorate the chocolate with cereal marshmallows, sprinkles, green sugar, edible glitter, or other decorations of your choice.

Set tray aside and allow chocolate to cool. Once chocolate has set, break apart or use a knife to cut into pieces.

Store chocolate bark in an airtight container at room temperature for up to one month.