Preparation
Preheat the oven to 350°F (175°C).
Line your baking sheets with parchment paper or silicone baking mats.
Mixing the Dough
In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar together until light and fluffy.
Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.
In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt.
Gradually add the dry ingredients to the butter mixture, stirring until just combined. Do not overmix.
If using, fold in the chocolate chips.
Forming the Cookies
Use a tablespoon or cookie scoop to scoop rounded balls of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Gently press a few mini marshmallows into the top of each cookie dough ball.
Baking
Bake for 10-12 minutes, or until the edges are set and the centers are still soft.
Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.