Begin by crushing Oreos. For a very fine mixture, place Oreos in a food processor. Transfer the crumbs to a bowl or zip-top bag, add the melted butter, and mix until evenly coated.
Press into the bottom of a 9x13 pan spreading out to cover the entire bottom of the pan. Chill in the refrigerator while preparing the next layer.
In a large bowl, beat the softened butter, cream cheese and powdered sugar until super smooth and creamy.
In a separate bowl, gently stir the Cool Whip and hot cocoa mix together until fully combined. Gently fold the hot chocolate Cool Whip mixture into the cream cheese mixture, making sure it’s fully combined and fluffy.
Spread the layer evenly over the crust and pop it in the fridge (or freezer) to firm up before adding the next layer.
In another bowl, whisk the cold milk and instant pudding mix together until thickened. Pour over the cream cheese layer. Refrigerate for at least 5 minutes.
Spread the remaining Cool Whip over the pudding layer. Refrigerate for at least 4 hours, or overnight, so the layers fully set. Just before serving, sprinkle with marshmallow bits. For clean slices, dip a sharp knife in hot water, wipe it dry, and clean it between cuts.