Prepare the crust: In a large mixing bowl, combine 15.5 oz of crushed Oreos and ½ cup melted salted sweet cream butter. Mix well until the crumbs are fully coated in butter. Press this mixture firmly into the bottom of a 9x13 inch dish. Make sure it's even; this will serve as the base for all your delicious layers. Chill in the refrigerator while you prepare the next layers.
Create the hot chocolate cheesecake layer: In a large mixing bowl, beat together 8 oz cream cheese and ½ cup softened butter until smooth and creamy. Add 1½ tsp vanilla extract and 1 cup powdered sugar and mix until well combined. In a separate bowl, whip 1½ cups heavy cream and 6 envelopes of hot chocolate mix together until stiff peaks form. Gently fold the whipped cream into the cream cheese mixture until combined. Spread this over the chilled Oreo crust and return to the refrigerator to set.
Prepare the chocolate pudding layer: In another bowl, combine 7.8 oz of instant chocolate pudding mix with 2¾ cups whole milk. Whisk together until it thickens, usually within 2 minutes. Once thick, fold in 1½ cups mini marshmallows for added texture. Spread this mixture evenly over the cheesecake layer and chill overnight to allow the layers to set properly.
Add the topping: After chilling overnight, take the lasagna out of the refrigerator. Spread 8 oz of whipped topping evenly over the top. Decorate with 2½ cups mini marshmallows and if desired, drizzle ¼ cup chocolate syrup on top for extra sweetness. Cut into squares and serve immediately!