Preheat your oven to 350°F. Line a baking sheet with parchment paper.
Whisk together the flour, salt, and baking soda.
In a medium bowl, beat together the butter, brown sugar, and granulated sugar until light and fluffy, about 2 minutes. Add the egg yolk and vanilla, beat until smooth and combined.
Add the flour mixture, mix until combined and a dough forms. If your dough is dry, your egg yolk was too small, add ½ teaspoon of milk at a time until the dough comes together.
Add the marshmallows and white chocolate, stir until combined. It will seem like too many add ins but keep stirring until the marshmallows and white chocolate is incorporated throughout the dough. I like to use my hands.
Portion the dough and roll into 6 balls. Place on the baking sheet.
Bake for 9-11 minutes, the edges will be set but the centers will look soft, they will continue baking on the cookie sheet. Do not over bake!
Use a round object like a cookie cutter or a small bowl to swirl around the edges of freshly baked cookies, creating perfectly round shapes. See storage in notes below.