Crush Oreo cookies using a rolling pin inside a sealed plastic bag until finely crumbled. Combine with melted butter in a mixing bowl and press into the bottom of a 9x13 glass pan. Refrigerate for 15 minutes to firm up.

In a microwave-safe bowl, combine mini marshmallows, milk, and peppermint extract. Microwave for 2-3 minutes, stirring every minute until smooth. Let cool for 40 minutes, stirring every 10 minutes.

Gently fold in Cool Whip into the cooled marshmallow mixture until fully incorporated without deflating the texture.

Chop Hershey chocolate bars into chunks and fold into the marshmallow filling evenly.

Pour the marshmallow-chocolate mixture over the Oreo crust and spread evenly. Top with Christmas sprinkles.

Cover with plastic wrap and refrigerate for 2-4 hours until set. Check for firmness after 2 hours.