Begin by crushing about 24 Oreo cookies into fine crumbs using a food processor or a rolling pin in a zip-top bag. In a mixing bowl, combine the cookie crumbs with ½ cup melted butter until fully blended. Press this mixture firmly into the bottom of a 9x13-inch glass pan to create a solid crust. Place the pan in the refrigerator for 10-15 minutes to set while you prepare the filling.

In a large microwave-safe bowl, add 4 cups of mini marshmallows, ½ cup of milk, and 1 teaspoon of peppermint extract. Microwave on high for 30 seconds, then stir. Continue melting in 10-second intervals, stirring after each, until the mixture is completely smooth and fluffy—this should take about 1-2 minutes total. Let the mixture cool in the fridge for up to 40 minutes, stirring every 10 minutes to keep it from hardening.

Once the marshmallow mixture has cooled and thickened, remove it from the fridge. In a separate bowl, fold in 8 ounces of Cool Whip gently until it's fully incorporated into the marshmallow mixture. After that, chop up 2 Hershey chocolate bars and mix them in to add a delicious chocolate flavor throughout your Peppermint Marshmallow No-Bake Dessert. The mixture should be light and airy.

Pour the creamy marshmallow filling over the chilled Oreo crust, spreading it evenly with a spatula for a smooth top. Add a festive touch by generously sprinkling Christmas sprinkles over the dessert, ensuring they cover the top evenly for a cheerful look.

Cover the assembled dessert with plastic wrap and refrigerate it for 2-4 hours, or until completely set. This will allow the flavors to meld beautifully while giving the filling a cloud-like texture. Before serving, check that it’s firm enough to be sliced into squares, ensuring a delightful presentation of your Peppermint Marshmallow No-Bake Dessert.