Line an 8x8-inch baking pan with parchment, allowing overhang on all 4 sides (this will prevent the chocolate from sticking and also make it easier to lift out of the pan once it sets).

Set a metal bowl over a pot of simmering water to create a double boiler. Add half the dark chocolate, 1/2 teaspoon vegetable oil, and 1/4 teaspoon peppermint extract. Stir until melted and smooth, then transfer to the prepared pan. Use a spatula to scrape the sides of the bowl, and make sure the chocolate is spread evenly in the pan. Place in the refrigerator to help set the layer.

Clean out the bowl using hot water. Add the white chocolate, 1/2 teaspoon vegetable oil, 1/4 teaspoon peppermint extract, and 3-4 drops food coloring. Place over the double boiler and stir until melted and smooth. Pour over the first layer of chocolate, spreading evenly. Place the pan back in the refrigerator to set.

Repeat step 2 using the remaining dark chocolate, vegetable oil, and peppermint extract. Place in the refrigerator and let the mixture set for at least one hour.

Remove the pan from the refrigerator. Use the parchment paper overhang to remove the chocolate from the dish, then let it come back to room temperature for 30 minutes. Cut into the desired size and serve. For cleaner cuts, briefly run the knife under warm water and wipe it clean before each cut. Press straight down with the knife versus using a sawing motion to help prevent the chocolate from breaking.