Preheat the oven to 375°F. Line 2 baking sheets with parchment paper or silicone baking mats. Whisk together the dry ingredients in a large bowl and set aside.
1 1/4 cup all-purpose flour, 3/4 cup cake Flour, 1 tsp. cornstarch, 1/2 tsp. baking soda, 1/2 tsp. baking powder, 1/2 tsp. salt
Cream butter and sugars: Place the butter and sugars in the bowl of a stand mixer. Secure the paddle attachment and blend until incorporated ~2-3 minutes. It wil look like a paste.
1/2 cup unsalted butter, cut into cubes, 1/2 cup brown sugar, packed, 1/4 cup granulated sugar
Then, add in the egg, egg yolk, and vanilla extract. Blend until mixed. If any of the mixture gets stuck on the paddle attachment or side of the bowl use a rubber spatula to knock it off.
1 large egg, 1 large egg yolk, 1 tsp. vanilla extract
Turn the mixer to low speed and gradually add in the dry ingredients. I add in about ¼ cup at a time mixing just until incorporated.
7 sheets graham crackers, broken into pieces (6 mixed in the cookie dough, 1 for garnish), 10 oz. Hershey's Chocolate Bar chunks (9 oz. mixed in the cookie dough, 1 oz. for garnish), 1 cup chocolate chips (milk chocolate or semi sweet)
Add in 6 graham crackers, Hershey's chocolate and chocolate chips until incorporated and then turn off the machine.
Measure out the cookies into ~7-8, ~5 oz. portions (or smaller if you prefer). Make a well in the center of the cookie with your thumbs. Slice a marshmallow in half and fold dough over marshmallow. Decorate cookies with reserved graham cracker pieces and chocolate chunks if desired. Place on prepared baking sheets.
4 marshmallows, cut in half
Bake at 375°F for 10-13 minutes or until tops are golden brown. Let cool for at least 15 minutes because they are very gooey straight from the oven. Garnish with flaky sea salt on top (optional) and enjoy!