Combine the flour, baking soda, and salt together in a large bowl and set aside.
In a medium bowl, using a mixer, mix the softened butter, brown sugar, and granulated sugar together until the mixture is smooth and there are no sugar lumps. Add the egg and egg yolk and vanilla extract.
Pour the wet ingredients into the dry ingredients and mix together until just combined.. Fold in the chocolate chips and 1 cup of the marshmallows. Stir to combine.
Cover the dough tightly and chill in the refrigerator for at least 30 minutes, or up to 2 hours.
Take the dough out of the refrigerator and allow it to slightly soften at room temperature for 10 minutes.
Preheat oven to 325°F and line a large baking sheets with parchment paper. Using a cookie scoop measure 1.5 Tablespoons of dough for medium-sized cookies and place them on the prepared baking tray. Don’t crowd the cookies! These will spread quite a bit so I’ve found it’s best to keep it to 6 cookies per tray.
Bake the cookies for 10 minutes, then take them out of the oven and gently press 2-3 marshmallows onto the tops of each cookie. Place back into the oven and cook for an additional 2 minutes until the edges are very lightly browned. Remove the cookies and sprinkle lightly with extra sea salt. The cookies will be soft but will continue to set as they cool.
Cool on the baking sheet for 10 minutes, then transfer to a cooking rack to cool completely. Don’t disturb the cookies as they are cooling!
Store the cookies at room temperature in a covered container for up to one week, or freeze in a freezer-safe container for up to 3 months.