Line a small plate or small baking sheet with parchment or wax paper. Portion out the peanut butter into 14 1 - 1.25" dollops. Pop in the freezer and freeze for about an hour before starting the cookies.*
Once your peanut butter is frozen, preheat oven to 350°F. Line 2 baking sheets with parchment paper or a silicone baking mat. Set aside.
Using a stand or hand mixer, cream together the butter and light brown sugar. Once light a fluffy (1-2 minutes), add in the eggs, egg yolk, and vanilla extract, and mix again until everything is combined.
Add in the flour, baking soda, and salt. Mix until everything until everything is incorporated and the cookie dough is formed. Fold in the chocolate chips.
Begin scooping cookie dough using a large cookie scoop. Use your thumb to press a cavity into the center of the scooped dough. Take a frozen dollop of peanut butter and press it into the middle of the cookie dough-filled scoop. Then, take a marshmallow, and gently press it on top of the peanut butter, just until the marshmallow set in the dough. Drop the filled cookie dough onto a piece of parchment paper. Repeat until all 14 cookies are scooped and stuffed.
Gently roll each cookie dough ball, just until they are round and the cookie dough partially covers the marshmallow. Place on prepared baking sheets, spacing cookies a few inches apart. Using your hand, lightly press down on the tops of the cookies to flatten then ever so slightly.
Bake for 11-14 minutes, until the cookies and marshmallows are just starting to change color on top.
Once baked, allow to rest on the cookie sheet for about 5 minutes. Transfer to cooling rack to let the cookies continue cooling. Enjoy warm or completely cooled. Leftover cookies will last for 4-5 days.