Preheat your oven to 350, then line two large baking sheets with parchment paper.
In a large bowl, cream together butter and sugars for 2 to 3 minutes until fluffy.
Beat in eggs, one at a time until incorporated, add vanilla, flour, baking soda and salt until just combined.
If the dough seems especially sticky, add flour 1 tablespoon at a time, then fold in the milk and dark chocolate chips.
Scoop 1/3 cup sized mound of dough into your palm and gently flatten to make a dent in the center.
Press marshmallow into the dent and fold dough around it. (It's okay if the mallow isn't fully covered.)
Roll the dough into a ball and set on the prepared baking sheet.
Repeat with the remaining dough. (Do not set more than 5 dough balls on your baking sheets, you may need to work in batches.)
Place the baking sheets in the freezer for 15 minutes, then remove and bake for 9 minutes.
Tap the baking sheets firmly on the edge of the counter until the centers flatten.
Bake 3 more minutes, and repeat the tapping.
Bake a final 3 minutes until the edges are golden and set, and perform a final tap on the counter.
Let the cookies cool on the baking sheets for 10 minutes, then transfer to a wire rack.
Eat some warm, and let the rest cool completely. Enjoy!