Begin by preheating your oven to 350°F (175°C). Line your baking sheets with parchment paper or silicone baking mats to prevent sticking and ensure even baking.
In a large mixing bowl, combine the softened butter, granulated sugar, and brown sugar. Use an electric mixer to beat them together until the mixture is light and fluffy, about 2-3 minutes.
Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Follow with the vanilla extract.
In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt.
Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed or folding gently with a spatula until just combined. Avoid overmixing.
Gently fold in the semi-sweet chocolate chips and mini marshmallows, being careful not to overmix to prevent marshmallows from melting prematurely.
Use a cookie scoop or spoon to portion the dough into 1 ½-inch balls and place them about 2 inches apart on the prepared baking sheets. Bake for 10-12 minutes, until edges are set but centers remain soft and marshmallows are slightly browned and gooey.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a cooling rack to cool completely.
