Preheat the oven to 350°F and line a baking sheet with parchment paper.
Cream the softened 1 cup salted butter, ¾ cup light brown sugar, and ¼ cup granulated sugar together in a stand mixer fitted with a paddle attachment.
Add the 2 large eggs one at a time, mixing for 15 seconds after each addition, then add 1 teaspoon vanilla extract.
Whisk together 2½ cups all-purpose flour, 1 tablespoon corn starch, and 1 teaspoon baking soda and add to the butter and sugar mixture in three parts, mixing after each on just until incorporated.
Add 1½ cups semi-sweet chocolate chips and ½ cup graham cracker crumbs and mix until incorporated.
Scoop 2 tablespoons of the cookie dough and roll it into a ball with your hands. Then flatten it into a dome shape (about 2-3 inches in diameter) on the prepared cookie sheet. Top the cookie with 5 to 7 1½ cups mini marshmallows and extra chocolate chips.
Bake the cookies for 10 to 12 minutes in the preheated oven. The cookies are done once they turn slightly golden, as they will continue to bake on the baking sheet as they cool.
Allow the cookies to cool on the baking sheet for about 10 minutes before transferring them to a serving platter.