Mix and bake cake according to the box directions. When a toothpick comes out clean, remove from oven and let sit for 5 minutes.
Place sweetened condensed milk in a small bowl. Stir in 1/2 teaspoon peppermint extract. (You can also do this in the can without transferring to a bowl, if you want to save a dish!)
Remove lid from jar of hot fudge sauce. Heat in microwave for about 30-60 seconds, until it is pourable. Be careful not to heat it too much without stirring, you don’t want it to burn. Stir in 1/2 teaspoon peppermint extract.
Once the cake has cooled for 5 minutes, poke several holes in the top with the back of the handle of a wooden spoon. Pour the sweetened condensed milk over the cake, making sure it gets in all the holes. Spread with a spatula if needed.
Pour the hot fudge sauce over the top of the cake, making sure to get it in all the holes. Spread with a spatula if needed. Cool the cake completely before continuing.
Open thawed whipped topping. Stir in 1/4 teaspoon peppermint extract and 3-4 drops green food coloring.
Spread over cooled cake and sprinkle with crushed Oreos. Chill for several hours (or overnight) before serving.
Best served room temperature.