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