Preheat the oven to 350°F. Spray a 9x13-inch baking dish with nonstick cooking spray.
Prepare the cake mix according to the package directions, adding in the mint extract, and bake per the directions on the back of the box.
When the cake comes out of the oven, take a wooden utensil handle or something similar and poke holes all over the cake, about 1 inch apart.
In a large microwave-safe bowl, add the marshmallow stuff and milk. Place in the microwave for 15 seconds, stir to combine until smooth.
Add half of the crushed peppermints and stir to combine.
Pour the marshmallow mixture over the cake. Take care to pour it right into the holes as much as possible. Spread it all out, and using the back of the spoon, gently push it down into the holes.
Place in the refrigerator for 2 hours to set.
Spread the whipped topping on top of the cake and place it in the fridge again for 2 hours or longer to set.
When ready to serve, add chocolate sauce and the remaining crushed peppermints on top.
Slice and serve. Keep cake refrigerated.
Notes
Please refer to my FAQ’s (Frequently Asked Questions) and ingredient list above for other substitutions or for the answers to the most common questions.