In the bowl of a stand mixer, or in a large bowl with an electric mixer, beat butter and sugars on medium speed for about 3 minutes. The mixture should become light and fluffy, which is crucial for our cookie's texture.
Add in vanilla and eggs and beat an additional 1-2 minutes. The mixture should be creamy and well-combined, with a rich vanilla aroma filling the kitchen.
In a medium bowl, whisk together flour, baking soda and salt. This step ensures that the leavening agent is evenly distributed, helping the cookies rise perfectly.
Gradually add in the flour mixture and mix until just combined. You want to be careful not to overmix; the dough should come together nicely with a few flour streaks still visible.
Now, stir in the chocolate chips and ¾ cup of the chopped graham cereal until they are evenly distributed throughout the dough.
Cover the bowl with plastic wrap and refrigerate the dough for at least 2 hours, or up to 24 hours. Chilling the dough helps the flavors meld and improves the cookie's texture.
When ready to bake, preheat your oven to 350°F (175°C). Line your baking sheets with pre-cut parchment paper to prevent sticking.
Using a medium cookie scoop (about one and a half tablespoons), measure out the dough and place the balls on the prepared cookie sheets, spacing them about two inches apart.
Bake for eleven minutes. Your cookies should look slightly underbaked but golden around the edges. Pull the pan from the oven and immediately top each cookie with mini marshmallows, pieces of graham cereal, and chunks of the milk chocolate bars.
Return the cookies to the oven and bake for an additional three minutes. The marshmallows should puff up and become soft.
Finally, allow the cookies to cool on the pan for five minutes before transferring them to a wire cooling rack to cool completely. Enjoy the warm, gooey cookies that remind you of summer nights!