Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the softened butter, white sugar, and brown sugar. Beat until light and fluffy, about 2-3 minutes.
Add the egg and vanilla extract to the butter and sugar mixture. Mix until well combined.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Gradually add to the wet mixture, mixing until just combined.
Gently fold in the chocolate chunks and mini marshmallows with a spatula.
Using a cookie scoop, drop rounded tablespoons of dough onto a lined baking sheet, leaving space between each.
Bake for about 10-12 minutes. Press pieces of Hershey chocolate bars into the tops of the warm cookies.
Allow cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
