1Preheat oven to 350°F (177°C). Line a baking sheet with parchment paper and set aside.
2In a large bowl, whisk together dark brown sugar, white sugar, and melted butter until smooth and glossy.
3Add eggs, vanilla extract, and milk to the bowl, mixing until fully combined.
4In a separate bowl, whisk together flour, salt, baking soda, baking powder, cornstarch, and cinnamon.
5Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.
6Gently fold in the chocolate chips.
7Scoop dough into 8–10 large portions. Lightly flatten each portion and place ½ sheet of graham cracker in the center. Top with 1 jumbo marshmallow and a piece of milk chocolate. Lightly cover with a bit of dough if needed.
8Place cookies on the prepared baking sheet, leaving space between each. Bake for about 13 minutes. The dough may look like it’s pulling away from the marshmallow.
9Remove from the oven and, while warm, gently shape the cookies using a round object if desired. Let cookies cool completely on the baking sheet. Do not move them too soon—they will set and hold together as they cool.