Line two large baking sheets with parchment paper and set them aside. Prepare your s'mores filling ingredients: cut each marshmallow in half, break graham crackers into smaller pieces by dividing each cracker into rectangles and then breaking those rectangles in half, and break the milk chocolate bars into pieces. Set all these aside for assembling the cookies later.

In a large mixing bowl, use a hand held mixer or a stand mixer to beat the softened butter, white sugar, and packed light brown sugar together until creamy. Add the eggs one at a time, beating after each addition, then add the vanilla extract. Continue beating the mixture until everything is well combined and fluffy.

In a separate large bowl, whisk together the all-purpose flour, baking soda, and salt. Gradually add these dry ingredients to the creamed butter mixture, mixing carefully and just until everything is combined, taking care not to overmix.

Gently fold the semi-sweet chocolate chips into the cookie dough using a large spatula. Use a 3-tablespoon cookie scoop to portion out 24 cookie dough balls. Place them on the prepared baking sheets (12 per sheet), then gently flatten each dough ball into a 3-inch disc.

Take 12 of the flattened cookie discs and place half a marshmallow, a couple pieces of broken graham cracker, and a couple pieces of broken chocolate bar on each. Be careful not to overfill. Place the remaining 12 cookie discs on top of the filled ones and gently press the edges together, then roll them to form a jumbo stuffed cookie ball. I like to be gentle with the edges so the filling doesn't leak out during baking.

Transfer all the stuffed cookie balls to one of the prepared baking sheets and place the sheet into your freezer. Freeze the cookie balls for 30 minutes to help them hold their shape during baking. Make sure the baking sheet stays flat in the freezer.

While the cookies are freezing, preheat the oven to 375°F. Place 4-6 frozen cookie balls on the parchment-lined baking sheets, leaving plenty of space between them since they'll spread during baking. Bake each batch for 13–15 minutes, or until the edges are golden and the centers are just set. Repeat with the remaining cookie balls. For an extra gooey center, I like to pull them out right at 13 minutes.

Allow the cookies to cool on the baking tray for 3-4 minutes before enjoying them. Be careful, as the gooey marshmallow insides will be very hot.