Instructions

• Fit a stand mixer with the paddle attachment (or use a large bowl and a hand mixer). Add the softened butter, light brown sugar, and granulated sugar to the bowl.
• Beat on medium-high speed until the mixture is light and fluffy, about 3–4 minutes. Add the egg and vanilla extract, then continue beating on medium-high until fully incorporated and the mixture is uniform and fluffy, about 1–2 more minutes (about 5 minutes total with a stand mixer; if using a hand mixer, beat at least 7 minutes total).
• Stop the mixer and scrape down the sides and bottom of the bowl with a spatula.
• Add the all-purpose flour, cornstarch, baking soda, and the pinch of salt (if using). Mix on low speed (or stir by hand) until the dry ingredients are just combined and no streaks of flour remain, about 1 minute. Do not overmix.
• Add the coarsely chopped graham crackers, semi-sweet chocolate chips, and marshmallow bits. Fold in by hand with a spatula until evenly distributed, or beat on low speed for just a few seconds to incorporate. Do not overwork the dough.
• Using a medium (2-inch) cookie scoop, portion the dough into heaping 2-tablespoon mounds. Place the mounds on a large plate or tray in a single layer. Slightly flatten each mound with your palm.
• Cover the dough mounds tightly with plastic wrap and refrigerate for at least 2 hours, or up to 5 days. Do not bake while the dough is warm; chilled dough prevents excessive spreading.
• When ready to bake, preheat the oven to 350°F (175°C). Line a baking sheet with a Silpat or parchment paper, or lightly spray with cooking spray. Place chilled dough mounds on the prepared baking sheet spaced at least 2 inches apart (about 8 cookies per standard sheet).
• Bake for 8 minutes. If baking multiple trays, rotate the baking sheet between racks at the 4-minute mark for even browning. The cookies are done when the edges are set and the tops are just beginning to set (centers may still look slightly soft, pale, and glossy). For soft, gooey centers do not bake longer than 9 minutes.
• Remove the baking sheet from the oven and allow the cookies to cool on the sheet for about 5 minutes. Transfer cookies to a wire rack to finish cooling.