01 - Preheat oven to 350°F and line two baking sheets with parchment paper.
02 - Whisk flour, baking soda, and salt in medium bowl. Set aside.
03 - Mix melted butter, brown sugar, and granulated sugar in large bowl until smooth.
04 - Beat in egg, then egg yolk. Add vanilla extract and mix well.
05 - Gradually add dry ingredients to wet ingredients, stirring until just combined.
06 - Gently fold in white chocolate chips and 1 cup marshmallows, reserving 1/4 cup for topping.
07 - Sop dough onto prepared baking sheets using tablespoon, spacing 2 inches apart.
08 - Lightly press reserved marshmallows onto top of each dough ball.
09 - Bake 9-11 minutes until edges are set and centers appear slightly underbaked.
10 - Let cookies cool on baking sheet for 5 minutes, then transfer to wire rack.