1-First, roughly chop the graham crackers and chocolate bar, then tear the mini marshmallows in half. This prep step ensures even distribution and helps the flavors meld. In a bowl, whisk together the all-purpose flour, salt, baking powder, baking soda, and cornstarch, then set it aside for later.

2-Next, in a separate large bowl, beat the unsalted butter, granulated sugar, and light brown sugar until creamy, which takes about 2 minutes. Beat in the egg and vanilla extract until everything is fully combined, creating a smooth base for your dough.

3-Gradually add the flour mixture to the butter mixture, beating on low speed until just combined to avoid tough cookies. Reserve a handful of the torn marshmallows and set them aside, then gently fold the remaining marshmallows, chocolate chips, and chopped graham crackers into the dough until just mixed.

4-Refrigerate the dough for at least 1 hour to firm it up, which prevents spreading. Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper for easy cleanup.

5-Portion the chilled dough into 2-tablespoon sized balls, roll them, and place them on the prepared baking sheets about 2 inches apart. Bake for 8 minutes, then remove from the oven and top each cookie with some of the reserved marshmallows and pieces of the chopped chocolate bar.

6-Return to the oven and bake for an additional 2-3 minutes until golden. If the marshmallows spread too much, use a round cookie cutter or knife to reshape them while warm. Let the cookies cool completely on the baking sheet to firm up, locking in that perfect texture.