Mix Dry Ingredients: In a medium bowl, whisk together flour, baking soda, and salt. Set aside.
Cream Butter & Sugars: In a large bowl, beat the butter, brown sugar, and granulated sugar until light and fluffy (about 2 minutes).
Incorporate Wet Ingredients: Mix in the egg, egg yolk, and vanilla extract until fully combined.
Combine: Gradually fold in the dry ingredients until just incorporated. Stir in the chocolate chips and ¾ cup of mini marshmallows.
Chill the Dough: Cover and refrigerate for at least 30 minutes (or up to 2 hours).
Preheat & Prep: Preheat oven to 325°F (163°C) and line baking sheets with parchment paper.
Shape & Bake: Roll dough into 1.5-tablespoon-sized balls and space them 2 inches apart on the baking sheet. Bake for 10 minutes.
Add Extra Marshmallows: Remove cookies from the oven, press 3-4 mini marshmallows into each, and return to the oven for 2-3 minutes, until edges are golden.
Cool & Enjoy: Let cookies cool on the baking sheet for 10 minutes, then transfer to a wire rack. Enjoy warm for the ultimate gooey treat!