To make the cookie dough: Whisk the flour, baking soda, and salt together in a medium bowl; set aside. In the bowl of a stand mixer fitted with the paddle attachment, or by hand, mix butter and sugars until thoroughly combined. Beat in the egg, yolk, and vanilla until combined. Add dry ingredients and beat at low-speed just until combined – the batter should look a little lumpy. Stir in chocolate chips.
Preheat oven to 325 degrees F. Line a baking sheet with parchment paper.
Divide the dough into 6 parts, spread 3 of them on the backing sheet in a shape of pancakes. Put a little bit of marshmallows on top of each and cover them with another layer.
Space the cookies at least 4 inches apart on the two baking sheets. Bake for 20-24 minutes or until the tops and edges are lightly browned and the edges just lift off the pan with a spatula.