Heat up the heavy cream in a pot on the stove until it boils, then remove it from the heat. Pour in the chopped semi-sweet chocolate, then stir the mixture thoroughly until the chocolate has melted.
Pour 1/3 of the chocolate mixture into a resealable plastic bag, transfer the bag to the fridge, and let the mixture cool for about 1 hour. Add the milk and corn starch to the remainder of the chocolate mixture, then bring it to a boil on the stove so that it thickens slightly. Once the mixture is the consistency of a thick pudding, remove it from the heat and let it cool completely.
Remove the plastic bag from the refrigerator, then slice the chocolate into thick circular pieces to make the XXL chocolate chips.
Combine the softened butter, sugar, and flour and knead it until a smooth dough forms. Wrap the dough up in plastic wrap, transfer it to the refrigerator, and let it chill for 90 minutes.
Place the XXL chocolate chips around a 12-inch oven-safe lid. Remove the cookie dough from the fridge and roll out 2/3 of it into a large circle, slightly larger than the lid. Place the rolled out dough on top of the XXL chocolate chips on the lid and cut off any excess dough.
Place a layer of marshmallows on top of the dough, followed by the 2 chocolate bars, slivered almonds, and the remainder of the chocolate ganache.
Roll out the remainder of the cookie dough into a large circle, then place it over the fillings and seal them in. Transfer the stuffed XXL chocolate chip cookie to the refrigerator and let it chill for 30 minutes.
Preheat the oven to 350°F, then transfer the lid to the oven and bake the XXL cookie for 2 hours.
Once the cookie has baked, remove it from the oven and let it cool for at least 2 hours.