In a large bowl, cream butter and sugar together, then add egg and vanilla extract. Mix to combine.
Add flour, baking soda, cocoa powder and salt. Mix well to combine.
Fold in chocolate chunks.
Roll dough into 14-16 1.5-inch balls, press 3-4 marshmallows onto the top of each of them, then place them on cookie sheet. Make sure to secure the marshmallow well if not it would fall out during baking.
Bake for 9-10 minutes. Check at the 9-minute mark. Once it is done (it will still be slightly soft), let cookies sit on baking sheet for 2-3 minutes before carefully removing them and placing them onto a baking rack to cool for 10 minutes. (See Note 1)
Serve and enjoy!
Note 1: Avoid over-baking cookies and also remove them from hot cookie sheet after 2-3 minutes once they are out of the oven so they do not continue cooking. This is to ensure cookies stay soft and moist!